feat(install): install without administrator rights (--no-admin)

Codeman itself always lived under $HOME; administrator rights were only
spent on what it needs first (system packages for Node.js, tmux and git,
Homebrew on a Mac). An account without those rights could not install it:
the installer died in ensure_sudo, or in Homebrew's own installer.

install.sh now has a no-admin route, taken by itself when the account has
no administrator rights (no sudo binary; outside sudo/wheel/admin and
`sudo -n true` fails; a refused sudo password; a Mac account outside the
admin group with no writable Homebrew) and forced with --no-admin or
CODEMAN_NO_ADMIN=1:

- Node.js: the official tarball from nodejs.org, checked against the
  SHASUMS256.txt published beside it, into ~/.codeman/runtime/node.
- tmux: the tmux project's own build (tmux/tmux-builds 3.7c), checked
  against a SHA-256 pinned in the installer, into ~/.codeman/runtime/bin.
- Both are linked from ~/.local/bin (never over a file that is not ours),
  and the runtime is on PATH and on the LaunchAgent's PATH.
- git is never installed. Without a working git the source comes as an
  archive, the install is marked with .install-source, and it updates by
  re-running the installer: the new tree is built beside the old one and
  swapped in only after the build succeeded. A later re-run on a machine
  that has gained git turns it back into a clone.
- Nothing on this route runs sudo, Homebrew or `xcode-select --install`,
  and the route is never decided by `sudo -v`.

What it cannot supply is checked before the first download: a C/C++
toolchain on Linux (node-pty 1.1.0 compiles there), musl, and macOS below
15 (the tmux build's deployment target).

Along the way:

- check_git no longer trusts `command -v git`. On a Mac without the Xcode
  Command Line Tools /usr/bin/git is a stub that opens Apple's install
  dialog; it is judged by `xcode-select -p` and never executed. Any other
  git is asked for its version.
- scripts/git-hooks.mjs answers "not a checkout" without running git, for
  the same reason: postinstall calls it on every npm install.
- The in-app updater knows the install kind `archive` and points such an
  install at the installer instead of `npm i -g aicodeman@latest`.
- download() and download_to_stdout() resolve DOWNLOADER themselves. Only
  main() used to set it, so `install.sh cloudflared` died on `set -u` with
  the message swallowed by the step log.
- The AI CLI menu leaves out a Homebrew-only install command where there
  is no Homebrew.

Verified with the real installer: a bare Ubuntu 24.04 container as an
unprivileged user (install, a PTY round trip through the API, update by
re-run, cloudflared, uninstall), the same without a compiler (stops before
any download), with a broken then working git, with sudo present but not
usable, and on macOS 26 (arm64) in an isolated HOME with sudo, brew, git,
the compilers, launchctl and tailscale replaced by tripwires. Not verified
on a real standard macOS account: that account was an administrator.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -69,6 +69,7 @@ This installs Node.js, tmux and a build toolchain if missing (node-pty ships no
 - **How it's reachable, your choice.** **Tailscale** (loopback bind fronted by `tailscale serve`, so you get `https://<machine>.<tailnet>.ts.net` with a real certificate and your tailnet as the login, no password needed), **any device on your network** (`0.0.0.0`, with a strongly recommended password prompt), or **this machine only** (`127.0.0.1`, safest). Skipping the password on a network bind requires an explicit confirmation and ends with a loud warning. The highlighted default reflects what is already on the machine (Tailscale when it is already connected, your existing binding on a re-run), and a bare Enter never pulls in new software. If another app already owns `:443` on your node, Codeman goes under `https://<machine>.<tailnet>.ts.net/codeman` or on a second port instead of replacing it. A bare `codeman web` started by hand still defaults to loopback.
 - **The name is yours to choose.** By default the URL uses the machine's existing tailnet name. Answering yes to the second question renames the machine to `codeman-<hostname>` (which also renames it for SSH, so the default is no); `install.sh name` does it later.
 - **Re-run to update.** The same one-liner updates a finished install in place: local changes in `~/.codeman/app` are stashed (never discarded), and a running service is restarted and verified. If a first install was interrupted, re-running resumes the full setup instead. `install.sh status` prints the URLs and the QR code again; `install.sh update`, `install.sh tailscale` and `install.sh uninstall` also exist.
+- **No administrator rights needed.** On an account without `sudo` (or a Mac account that is not an administrator), the installer downloads Node.js from nodejs.org and the tmux project's own build into `~/.codeman/runtime`, each checked against its SHA-256, and takes the source as an archive when there is no working git. `--no-admin` forces that route. Two limits: Linux needs make, a C++ compiler and python3 to be there already, and the tmux download needs macOS 15 or newer. Details in the [wiki](https://github.com/Ark0N/Codeman/wiki/Installation#without-administrator-rights).
 - **Flags for the impatient.** `curl -fsSL https://getcodeman.com/install | bash -s -- --tailscale --service` answers the questions from the command line (`--lan`, `--local`, `--run`, `--no-start`, `--name <n>`, `--port <n>`, `--yes` too). **CI / headless:** without a terminal attached, steps that would change your system abort with instructions instead of running silently; set `CODEMAN_NONINTERACTIVE=1` to approve them for automation.
 
 You'll need at least one AI coding CLI installed — [Claude Code](https://docs.anthropic.com/en/docs/claude-code), [OpenCode](https://opencode.ai), [Codex](https://developers.openai.com/codex/cli), [Antigravity](https://antigravity.google), [Gemini CLI](https://github.com/google-gemini/gemini-cli), [Pi](https://pi.dev), [Grok Build](https://github.com/xai-org/grok-build), [DeepSeek Harness](https://github.com/deepseek-ai/deepseek-harness), [OMP](https://github.com/can1357/oh-my-pi), or [GitHub Copilot CLI](https://github.com/github/copilot-cli) (any combination works; Gemini CLI is enterprise-only since Google's consumer cutover, and Antigravity is its successor). The installer detects whichever of the ten is present; if none is found, it offers to install any of them from a menu (DeepSeek excepted, since its npm package installs only a launcher with no runnable profile), or you can skip and install one yourself later. After install: