test(daemon): e2e regression guard for spawn cwd stability

[illegalcall]

Summary

Adds a small end-to-end regression guard for the daemon spawn working-directory
fix (#2871, RCA #2780 / symptom #2775). It runs the real ao daemon and
real tmux, mirroring the style of test/cli/install-check.sh.

test/daemon/cwd-stability-check.sh asserts:

1. Daemon self-stabilizes. Launched from a directory that is then deleted
   (the Squirrel/ShipIt "poisoned cwd"), the daemon stays healthy, has chdir'd
   to its data dir, and preserves its startup cwd in /healthz
   (startupWorkingDirectory).
2. Relative AO_DATA_DIR is absolutized. No reldata/reldata double-nest
   after the daemon chdirs.
3. tmux cd <ws> || exit guard. The pane lands in the workspace even
   against a tmux server whose own cwd was deleted.

Each daemon is discovered via its own running.json, so the check is immune to
port collisions (the daemon falls back to an ephemeral port when its configured
one is taken). The tmux check skips cleanly when tmux is absent.

Test

$ go build -o ./ao ./backend/cmd/ao
$ AO_BIN=./ao test/daemon/cwd-stability-check.sh
...
RESULT: 11 passed, 0 failed

Notes

• Not wired into CI here — it's a manual/opt-in check for now. Happy to add a
  workflow step (build ao, run the script) if wanted.
• Honest caveat baked into Test 3: it asserts the guard produces the correct
  cwd; the -c-ignored tmux failure mode it defends against is
  version/OS-specific and does not reproduce on every host.

🤖 Generated with Claude Code

[somewherelostt]

Thanks for contributing to Agent Orchestrator.

This PR is being picked up by the current external contributor on-call pair:

• @illegalcall
• @Pulkit7070

If someone is already working on this, please continue as usual.
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