feat(desktop): add a coding agent once Codeman is installed

The agent chooser only appeared during the install, and only on a Mac with
no agent. After "Skip for now", or with Codeman already installed, there was
no screen for it.

The same chooser now works after the install: "Add a coding agent" on the
This Mac tab, "Add a Coding Agent to This Mac..." in the app menu (it also
works from a Codeman page), and after a Skip the app says Codeman has no
agent yet before it opens Codeman.

`install.sh agent <id>` installs one agent and nothing else. It checks that
THAT agent landed: install_offered_cli is fatal only when no CLI is left, so
with another one present a failed install would have reported success.

The app runs it as a run of its own (`gui_install::Run::Agent`), in its own
temporary folder, so `ps` never reads it as "Codeman is being installed". It
may run next to a live server, which an install must refuse, and stops only
for another run. `--no-admin` goes to both `preflight` and `agent`: it decides
what counts as installable, so the two must agree.

CI: the bash 3.2 preflight check never tested anything. Its child run
inherited CODEMAN_INSTALL_SH_LIB and stopped at the sourcing guard, and a
false [[ ]] does not trip set -e on bash 3.2. Both are fixed for that check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -1303,7 +1303,7 @@ On this route Node.js comes from nodejs.org (`latest-v<major>.x/SHASUMS256.txt`
 
 ⚠️ `no_admin_blocker` runs before the first download and stops for what this route cannot supply: a C/C++ toolchain on Linux (node-pty 1.1.0 compiles there; a node-pty with Linux prebuilds is what would lift this, and it is a dependency decision, not an installer one), musl, a platform nodejs.org or tmux-builds has no build for, and macOS below 15 (the tmux build's deployment target). ⚠️ Nothing between the "No-admin route" header and "End of the no-admin route" may run `sudo`, `ensure_sudo`, `run_as_root`, Homebrew or `xcode-select --install`; `run_as_root` itself refuses while `NO_ADMIN=1`, so a missed call site fails instead of prompting.
 
-**Driving the installer from a program** (`--ai-cli`, `--progress`, `install.sh preflight`, 2026-11-06). Codeman Desktop installs with no terminal: it runs `install.sh --no-admin --service --progress --ai-cli <id>|none` with `CODEMAN_NONINTERACTIVE=1` and its stdin closed, and asks the two questions in its own window (`packages/desktop/src/gui_install.rs`). `--ai-cli` answers the agent menu, so the one thing `CODEMAN_NONINTERACTIVE` used to decide by itself (the first entry of the catalogue) is the user's. ⚠️ `cli_offerable` is the ONE rule for what can be installed here: the menu, `--ai-cli` and `preflight`'s `installable` all read it, so the app's list and the menu cannot disagree. A name that is not offerable here warns and carries on, and never falls back to another CLI; a typo dies in `parse_flags` before anything runs. ⚠️ `preflight` prints only JSON on stdout and changes nothing; `schema` moves only when a field changes meaning, because the app ships on its own and refuses a number it does not know. Fields may be added freely. ⚠️ `progress_phase` writes to stderr, not stdout, because a function can run inside a `$( )` that captures stdout. The phases must stay in the order of `PHASES` in `connect.html` (pinned by test), `done` must stay the last thing `main()` reports, since a run that ends without it failed, and an unknown phase is ignored by the app, so a new phase needs no new app. ⚠️ `error()` carries the text as `@@codeman error`, and the app shows the FIRST one: the ones after it are consequences (`Installation failed. Partial...`).
+**Driving the installer from a program** (`--ai-cli`, `--progress`, `install.sh preflight`, 2026-11-06). Codeman Desktop installs with no terminal: it runs `install.sh --no-admin --service --progress --ai-cli <id>|none` with `CODEMAN_NONINTERACTIVE=1` and its stdin closed, and asks the two questions in its own window (`packages/desktop/src/gui_install.rs`). `--ai-cli` answers the agent menu, so the one thing `CODEMAN_NONINTERACTIVE` used to decide by itself (the first entry of the catalogue) is the user's. ⚠️ `cli_offerable` is the ONE rule for what can be installed here: the menu, `--ai-cli` and `preflight`'s `installable` all read it, so the app's list and the menu cannot disagree. A name that is not offerable here warns and carries on, and never falls back to another CLI; a typo dies in `parse_flags` before anything runs. `install.sh agent <id>` installs one CLI and nothing else, for Codeman Desktop's *Add a coding agent*: no service, no source, no build, so it may run next to a live server, and an agent already there is a success that runs nothing. ⚠️ It checks `CLI_FOUND_PATH` of that one entry itself and only then prints `done`: `install_offered_cli` is fatal only when NO CLI is left, which is right for a first install and would report success here, where another CLI is usually present. ⚠️ The app passes `--no-admin` to both `preflight` and `agent`, because that flag decides what `cli_offerable` allows; pinned in `gui_install.rs`. ⚠️ The app runs it from a folder of its own (`Run::Agent`), so `ps` never reads an agent run as "Codeman is being installed". A running server remembers a missing CLI for up to 5 minutes (`cliResolveRetryDelayMs`), and only the CLI management routes clear that, so the app says so on its last screen and never restarts the service for it. ⚠️ `preflight` prints only JSON on stdout and changes nothing; `schema` moves only when a field changes meaning, because the app ships on its own and refuses a number it does not know. Fields may be added freely. ⚠️ `progress_phase` writes to stderr, not stdout, because a function can run inside a `$( )` that captures stdout. The phases must stay in the order of `PHASES` in `connect.html` (pinned by test), `done` must stay the last thing `main()` reports, since a run that ends without it failed, and an unknown phase is ignored by the app, so a new phase needs no new app. ⚠️ `error()` carries the text as `@@codeman error`, and the app shows the FIRST one: the ones after it are consequences (`Installation failed. Partial...`).
 
 ⚠️ **git is judged by whether it works, never by `command -v`.** On a Mac without the Xcode Command Line Tools `/usr/bin/git` is a stub that opens Apple's install dialog, so `check_git` judges that path by `xcode-select -p` and never executes it; any other git is asked for `--version`. `scripts/git-hooks.mjs` has the matching guard (no `.git`, no `git` call), because postinstall runs it on every `npm install`. Without a working git the source is an archive (`fetch_source_archive`, GitHub's `archive/<ref>.tar.gz` or `CODEMAN_TARBALL_URL`) and the install directory carries the marker `.install-source`, which `detectInstallKind` reads as `archive` (the name is pinned across both files by `test/install-sh-invariants.test.ts`). ⚠️ An archive install is replaced as a WHOLE, never patched in place: nothing says which files a newer version dropped, and a stale `src/*.ts` would be compiled. The new tree is fetched and built in `<install dir>.new` and `finish_source_swap` moves it into place only after `npm run build` succeeded, so a failed update leaves the running install untouched; a re-run on a machine that has gained a working git stages a clone instead, which turns the install back into a git one. ⚠️ `download`/`download_to_stdout` resolve `DOWNLOADER` themselves (`require_downloader`): only `main()` used to set it, so a subcommand that downloads died on `set -u` with the message swallowed by the step log.
 

docs/wiki/Installation.md

@@ -79,6 +79,7 @@ install.sh tailscale    # retrofit Tailscale access onto an existing install
 install.sh name [<n>]   # rename this machine on your tailnet (default codeman-<hostname>)
 install.sh cloudflared  # install cloudflared for the in-app Cloudflare tunnel
 install.sh preflight    # print what is on this machine as JSON; changes nothing
+install.sh agent <id>   # install one agent CLI and nothing else, also next to a running Codeman
 ```
 
 **Flags** answer the questions from the command line and pipe through `bash -s --`:
@@ -97,6 +98,10 @@ existing install, `--port` and `--password` re-run the setup so the service file
 and a re-run with `--lan` or `--tailscale` keeps the password the service already has.
 `--ai-cli <id>` answers the agent menu that appears when no agent CLI is found, with no
 terminal needed (`--ai-cli claude`, `--ai-cli codex`, ...), and `--ai-cli none` skips it.
+To add an agent later, `install.sh agent codex` installs that one and changes nothing else:
+it does not touch Codeman or its service, and an agent that is already there is left alone.
+Codeman Desktop does the same from *Add a coding agent*. A running Codeman can take a few
+minutes to notice the new agent; reload the page if it does not show up.
 
 **Automation and CI**: with no terminal attached, any step that would change the system
 aborts with instructions instead of running silently. Set `CODEMAN_NONINTERACTIVE=1` to