Move .polyphony/ ops state to git common-dir + add OS file lock (umbrella)

[PolyphonyRequiem]

Problem (umbrella issue — supersedes the two below)

.polyphony/ operational state (run.yaml, run.lock) currently lives at the worktree top-level, but the apex-driver fans out work across multiple worktrees off the same root branch. This creates two coupled bugs:

Sub-bug A: RunLockPathResolver resolves to worktree top, not git common-dir

RunLockPathResolver.ResolveAsync calls IGitClient.GetTopLevelAsync(), which returns the worktree top-level — meaning each worktree gets its own lock and the same root can be acquired N times concurrently. The lock provides no actual protection under fanout.

Sub-bug B: Manifest write-coordination under worktree fanout

RunManifest at .polyphony/run.yaml is worktree-local until merged. Today writes serialize through feature/{root_id} so works-in-flight don't clobber each other, but this is implicit and easily broken.

Proposed structural fix

1. Site shared ops state at <git-common-dir>/polyphony/<root_id>/. (git rev-parse --git-common-dir returns the shared .git even from a worktree.)
2. Add an OS-level file lock (fcntl.flock on Linux/macOS, LockFileEx on Windows) on the manifest path.
3. Migrate RunLockPathResolver and RunManifest to the new location simultaneously.

This collapses two sub-bugs into one structural change.

References

• PR docs(adr): branch-model Rev 4.2 — relocate per-root state to git common dir #251 / feat(state): relocate run lock to git common dir (Rev 4.2 — Stages 1-3) #252 / feat(manifest): root-id-aware path resolution (Rev 4.2 — Stage 4) #254 / feat(manifest): drop git-tracked manifest, flip readers/writers to local file (Rev 4.2 — Stages 5+6+7) #255 (Rev 4.2 state-location migration — established the pattern this issue extends)
• Memories: .polyphony ops state is per-root, gitignored, never tracked

Subsumed scope

This issue intentionally subsumes:

• "Fix RunLockPathResolver to use git common-dir not worktree top-level"
• "Decide manifest write-coordination story under worktree fanout"

Track those as two checkboxes inside this issue rather than as separate filings — the structural fix is one PR.