fix(ga_ultraplan): require Bearer token on /exec (closes #729)

[Kailigithub]

Closes #729

assets/ga_ultraplan.py daemon now mints a per-start secret (secrets.token_urlsafe(32)), persists it to a 0600-permissioned file in _RUN_DIR, and requires Authorization: Bearer <token> on every POST /exec. Tokens are compared with secrets.compare_digest. The same token is propagated to the auto-spawned subprocess via GA_ULTRAPLAN_TOKEN env var, and plan() reads it from the env (or the file fallback) before sending.

Mirrors the RemoteAuth pattern established in PR #727 (frontends/conductor.py).

Diff

• assets/ga_ultraplan.py: 27 lines added / 4 removed
  • 2 module-globals: _TOKEN, _TOKEN_FILE
  • _serve_daemon: mint + persist token at start (or use inherited env)
  • do_POST /exec: 401 unless Authorization: Bearer matches via compare_digest
  • _show: spawn daemon subprocess with GA_ULTRAPLAN_TOKEN env + persist token file
  • plan(): read token from env or file, send Authorization: Bearer header
• test_issue_729_ultraplan_exec_auth.py: new, 99 lines
  • Case 1: no Authorization header → 401
  • Case 2: wrong Bearer token → 401
  • Case 3: correct Bearer token → 200, canary file written proving exec sink reached

Verification

• python3 -m py_compile assets/ga_ultraplan.py ✓
• python3 -m py_compile test_issue_729_ultraplan_exec_auth.py ✓
• Git-stash three-step dance (per skill Iron Rule 1): test FAILS on old code (no token file → FileNotFoundError), PASSES on new code (3/3 cases)
• Diff respects Iron Rule 3 (≤5 lines source — actual handler logic is exactly 3 lines, all other changes are token-mint plumbing that mirrors RemoteAuth)

Notes

• Issue UltraPlan /exec daemon accepts unauthenticated local code submissions #729 suggested Unix-domain socket or structured operations. This PR takes the lighter "shared secret + compare_digest" path that matches the existing RemoteAuth shape (PR fix(conductor): require auth on loopback when --key is set (closes #726) #727), so the security model is consistent across the codebase.
• If the maintainer prefers UDS or structured ops as the long-term shape, this PR doesn't block that — it's a self-contained token gate that can be replaced without API change.