Agent Linux AppImage cannot spawn its host: current_exe() is the bundled loader #1207

The packaged Linux AppImage cannot start its own host process. AppRun execs the bundled loader (linux-release-appimage.py), so inside the app /proc/self/exe, and therefore std::env::current_exe(), is ld-linux-x86-64.so.2 rather than maple-agent. spawn_host() runs current_exe() with host serve, so the loader tries to load a program named host.

Package: maple-agent-prod-0.1.0-e20a5b7-linux-x86_64.AppImage, source e20a5b78c96aece2c081dfd624c80c8cd41f37d9, from master build 39659041169. Its SHA-256 matches the package's SHA256SUMS.

Environment: Ubuntu 24.04 x86_64, fresh XDG config/data roots, APPIMAGE_EXTRACT_AND_RUN=1, no display.

Reproduction:

1. ./maple-agent-prod-0.1.0-e20a5b7-linux-x86_64.AppImage remote status connects with start = true, so it spawns the host. It fails with Maple host did not start. See the profile's maple-agent.log. (exit 1); the log contains only the CLI's own start line.
2. What the spawn effectively runs, squashfs-root/usr/lib/ld-linux-x86-64.so.2 host serve, fails with host: error while loading shared libraries: host: cannot open shared object file: No such file or directory (exit 127).
3. Control: after starting ./…AppImage host serve in the background, ./…AppImage remote status connects and returns the expected signed-out answer, Sign in before using remote access; host stop exits 0.
4. Mechanism: /lib64/ld-linux-x86-64.so.2 /usr/bin/readlink /proc/self/exe prints the loader's path.

Impact: the desktop always attaches to the host as a client and spawns it on demand through the same path (backend.rs → connect(…, true)), so the packaged Linux desktop most likely cannot run tasks without a manually started host. The GUI itself was not run here; #1166 used an explicit host serve to get past the same self-spawn failure. The incompatible-host "Stop existing host and reopen" action relaunches current_exe() as well, and gpui's Linux restart() defaults to current_exe(), which matters for a future updater.

CI gap: Linux package verification runs only --version and --build-info; nothing spawns the host from the packaged AppImage.

Fix direction: inside an AppImage (APPIMAGE set and the running binary under APPDIR), use $APPIMAGE as the program for host spawns and relaunches; otherwise keep current_exe(). Spawning through $APPIMAGE also gives the host its own mount instead of the GUI process's mount, which goes away when the GUI exits. Add a packaged-AppImage check that spawns the host, for example remote status reaching the sign-in guard.

Related: #1166.