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/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