feat(bin): add opt-in host-root supervision

[YrFnS]

Intent

Keep FirstMate operable from an opt-in external instruction root (FM_HOST_ROOT) while preserving normal unset behavior, isolated target-worktree workers, native Windows Herdr support, endpoint identity, teardown safety, and SecondMate recovery.

What Changed

• Added opt-in host-root supervision and host-owned task/session authority.
• Launches ordinary workers from isolated FM_TARGET_WORKTREE checkouts; primary Pi/OpenCode host plugins stay inert inside target workers.
• Binds tmux lifecycle actions to the recorded cwd/socket/marker instead of ambient tmux state.
• Binds Herdr lifecycle actions to the exact workspace/tab/pane/task-label relationship, uses nonce-scoped cwd probes, and revalidates immediately before cleanup.
• Makes authorized host relaunch and SecondMate recovery transactional, including restoration of all prior per-task artifacts after failed recovery.
• Stops and verifies endpoints before returning target worktrees.
• Extends changed-source test routing so host-root, spawn recovery, and every OpenCode primary plugin select their behavioral regressions.

Safety Fixes Included

This update closes all confirmed review and audit blockers:

1. Recorded tmux authority for control and foreground inspection.
2. Authorized host-root relaunch without wrong-root takeover.
3. Transactional SecondMate metadata/status/turn-end/harness recovery.
4. Nonce-bound native Windows Herdr cwd polling.
5. Origin-host authority for decision decline/repair.
6. fm_tmux_cli use for tmux foreground probes.
7. Pi/OpenCode primary-plugin isolation in target workers.
8. Home-wide host session authority for unset or conflicting roots.
9. Herdr workspace/tab/pane/task-label ownership validation.
10. Immediate Herdr ownership recheck before close and stop-before-return ordering.
11. Relaunch authority validation before fm-guard.sh can mutate supervision state.
12. Refusal to open host-A records from host B.
13. Conservative changed-test routing with independent plugin-path regressions.

Risk Assessment

High-touch lifecycle change, mitigated. The code affects spawn, control, recovery, and cleanup boundaries, so regressions could otherwise target the wrong endpoint or return an in-use worktree. The implementation now fails closed on identity/authority mismatch and has behavioral regressions at each destructive boundary. No known implementation blocker remains from the completed reviews.

Validation

Validated at head 7c8d4675f5ea0577421944f09f641bcdb7c9c7c0, rebased onto origin/main f1a4af426d7199c1781bc91ccd143b8e1f732d10:

• bash bin/fm-lint.sh
• bash bin/fm-doc-audience-check.sh
• bin/fm-test-run.sh --check-coverage
• git diff --check
• Bash syntax checks for every modified shell file
• Focused host control/relaunch, host authority, Pi/OpenCode isolation, backend stop verification, Herdr binding/cwd/reuse, SecondMate recovery, teardown, and decision-lifecycle regressions
• Full tests/fm-secondmate-liveness.test.sh
• Full tests/fm-decision-hold-lifecycle.test.sh
• Canonical targeted SecondMate, decision-lifecycle, Pi-plugin-isolation, lint, and documentation-audience families
• Two independent whole-diff Pi reviews plus a final routing review; all actionable findings were fixed

The full local suite still encounters pre-existing MSYS/Windows fixture limitations also reproduced on the baseline (symlink handling, CRLF-sensitive Herdr output, process ancestry/ps, date-sensitive fixtures, and the existing fm-test-run jobs-scheduling fixture). Hosted GitHub workflows are pending upstream maintainer approval.

Merge

This PR has not been merged. Please approve and run the hosted workflows before merge.

[kunchenguid]

Automated reminder: thanks for the PR! This branch currently has a merge conflict with the base branch.

When you get a chance, please rebase onto (or merge) the latest base branch, resolve the conflict, and push. After that, checks will re-run and the PR will get looked at again.

_{Noted for firstmate#1238 at eb42af2e.}

[kunchenguid]

Automated reminder: thanks for the PR! This branch currently has a merge conflict with the base branch.

When you get a chance, please rebase onto (or merge) the latest base branch, resolve the conflict, and push. After that, checks will re-run and the PR will get looked at again.

_{Noted for firstmate#1238 at 39e72446.}

[kunchenguid]

Automated reminder: thanks for the PR! This branch currently has a merge conflict with the base branch.

When you get a chance, please rebase onto (or merge) the latest base branch, resolve the conflict, and push. After that, checks will re-run and the PR will get looked at again.

_{Noted for firstmate#1238 at baa5ef04.}

[YrFnS]

Rebased this PR onto current main and updated the host-root topology: the external host remains the supervisor authority, while ordinary ship/scout workers launch in their isolated target worktrees and load target-repository instructions natively. FirstMate briefs, completion signaling, recovery metadata, teardown safety, and SecondMate isolation remain intact.

Validation at head 929cbb4c passed no-mistakes review, focused host-root/Orca/real-Herdr checks, the complete behavior suite, documentation checks, and pinned ShellCheck lint. One unrelated Calm/Pi 12-second notification wait was intermittent; it passed in the preceding full run and on immediate isolated retry.

The new upstream Actions runs are currently action_required with no jobs, so they need maintainer approval before CI can execute:

• CI: https://github.com/kunchenguid/firstmate/actions/runs/30697737213
• Require no-mistakes: https://github.com/kunchenguid/firstmate/actions/runs/30697737190