fix: use evidence-based primary scope detection

[vipentti]

Intent

Prepare the upstream PR for issue #1809 , OpenCode linked-worktree supervision: use evidence-based primary detection instead of gitDir==commonDir. Rebase the recorded fork branch onto latest upstream main, preserve only the evidence-based predicate changes, the once-only not-primary diagnostic, and their direct callers, tests, and docs. Validate with the focused primary-scope coverage in tests/fm-turnend-guard.test.sh, tests/fm-pi-watch-extension.test.sh, shell lint on changed bin scripts, and documentation checks. Run the full no-mistakes flow with CI intentionally skipped under standing project policy, then push to the fork and open the upstream PR against kunchenguid/firstmate. Do not merge or push the default branch. Keep the validator agent and its codex/gpt-5.6-luna configuration unchanged; do not change no-mistakes configuration or shared daemon. Preserve SSH-signed commits under Ville Penttinen captain identity with no co-author trailers, AI attribution, machine-specific paths, hostnames, account IDs, or incident task names.

What Changed

• Replaced git topology-only primary detection with shared lock and active-state evidence, including --prelock handling for session-start callers and inherited-home rejection for child worktrees.
• Updated OpenCode watcher supervision with shared scope checks and once-only not-primary diagnostics.
• Updated focused tests and primary-scope documentation.

Pipeline

Updates from git push no-mistakes

✅ **intent** - passed

✅ **Rebase** - passed

✅ **Review** - completed

✅ **Test** - passed

✅ **Document** - passed

✅ **Lint** - passed

✅ **Push** - passed