feat: add optional no-mistakes gate mode

[JTInventory]

Summary

• Adds optional +nm-gate support while keeping direct-PR as the base delivery mode.
• Records no-mistakes gate metadata in task state so Firstmate can display and route it without changing scout/report behavior.
• Updates briefs so workers do not run no-mistakes themselves and stop at Firstmate PR scope review.
• Makes PR metadata updates idempotent and requires explicit no-mistakes passed status.

Behavior

• direct-PR remains the base mode.
• +nm-gate is optional and orthogonal to +yolo.
• Scouts ignore nm_gate and keep nm_status=not_applicable.
• Workers do not run no-mistakes themselves for direct-PR +nm-gate.
• Workers stop at done: ready for Firstmate PR scope review.
• nm_status=passed must be explicit; no-mistakes PR source alone records nm_status=pr_recorded.
• data/projects.md is not changed in this PR.
• Local fleet config can be updated after merge.

Tests

• git diff --check
• bash -n bin/fm-project-mode.sh bin/fm-brief.sh bin/fm-spawn.sh bin/fm-pr-check.sh bin/fm-fleet-sync.sh bin/fm-home-seed.sh tests/*.test.sh
• bash tests/fm-project-mode.test.sh
• bash tests/fm-brief-nm-gate.test.sh
• bash tests/fm-spawn-nm-gate.test.sh
• bash tests/fm-pr-check.test.sh
• bash tests/fm-spawn-batch.test.sh
• bash tests/fm-teardown.test.sh
• bash tests/fm-secondmate.test.sh

Post-Deploy Monitoring & Validation

No additional operational monitoring required. This change only updates Firstmate local workflow scripts, documentation, and shell tests; it does not change a live service or runtime process.

[JTInventory]

Maintainer update from JTInventory, 2026-06-25:\n\nThis PR is still conceptually useful, but it is not ready to merge as-is. It now conflicts with the newer Firstmate workflow changes on main, especially around AGENTS/README and the brief/spawn flow.\n\nRecommended handling:\n- Do not merge until rebased/refreshed.\n- Keep it as the optional no-mistakes gate-mode proposal.\n- Rebase after the current branch-metadata hardening PR is settled, because that PR changes the same branch/brief assumptions this PR touches.\n- Preserve the core boundary: direct-PR remains the base mode; the no-mistakes gate should be optional and explicit, not silently applied to scouts or local fleet state.\n\nI can refresh this branch after the branch-metadata PR lands, so the owner does not have to resolve the stale conflicts manually.