feat(dispatch): reapply Antigravity quota preflight before spawn

[Ospeto]

Summary

Reapplies the Antigravity quota-preflight track onto upstream firstmate (origin/main), restoring the deterministic preflight check before task worktree, backend endpoint, or task metadata creation.

Upstream Comparison & Changes

• bin/fm-spawn.sh:
  • Added header documentation detailing the Antigravity quota preflight contract.
  • Tracked raw launch command invocations (RAW_LAUNCH=1).
  • Implemented process-group bounded antigravity_preflight() using native Perl process-group supervision (setpgrp(0,0), pipe streaming, byte limits, and alarm cancellation).
  • Exit 0 preserves requested model/effort. Exit 1 (exhausted accounts) updates model to cockpit/gpt-5.6-luna at high effort (and replaces or appends raw launch flags on raw launch). Unrecognized exit codes or timeout exit 124 refuse spawn with secret-safe diagnostics.
• docs/architecture.md & docs/configuration.md:
  • Added authoritative specification for Antigravity quota preflight under Dispatch Profiles.
• tests/fm-spawn-dispatch-profile.test.sh:
  • Added targeted tests (totaling 43 tests) covering exit 0, scout profiles, exit 1 Luna fallback, raw launch embedded replacement, secret-safe errors, missing/non-executable checker, process-group timeouts, forked checkers, large output (>8KB), space-containing state paths, and non-Antigravity bypass.

Review & Verification

• Obtained read-only @oracle review of the diff and test evidence (Oracle approved).
• All 43 tests in tests/fm-spawn-dispatch-profile.test.sh passed cleanly.
• tests/fm-lint.test.sh passed clean.
• git diff --check clean.