fix(auto-sync): reliable cross-platform post-commit auto-update

[RaghavChamadiya]

Summary

The post-commit hook installed by repowise hook install was racing with itself: concurrent repowise update invocations from rapid commits all started from the same stale base, took 12+ minutes each, never converged, and discarded all output to /dev/null. The augment staleness warning then fired noisily for every tool call while zombie updates piled up in the background.

This PR makes auto-sync reliable on Windows, macOS, and Linux.

Changes

• Single-flight enforcement. Both repowise update (single-repo) and the workspace update path check .repowise/.update.lock before starting. If another update is running, the new invocation writes .update.pending with the current HEAD and exits cleanly; the running update rolls forward to that HEAD when it finishes.
• Queued marker before background spawn. The hook writes .update.queued synchronously, then backgrounds the heavy work. The augment hook treats the queued marker the same as a held lock, closing the 1–5s race window during Python start-up.
• Better augment messaging. When a lock or queued marker is present, the agent sees Wiki update in background — started Ns ago, target X instead of Wiki is stale. Falls through to the stale warning only when no update is in flight (or the marker is past its staleness window: 30 min for the lock, 5 min for the queued marker).
• Diagnosable hook output. Hook captures stdout/stderr to .repowise/.update.log (rotated to a 64 KB tail when it exceeds 256 KB) instead of /dev/null.
• In-place hook upgrade. When the marker block exists but its body differs from the current _HOOK_SCRIPT, install replaces the block in place (preserving surrounding hook content) and returns upgraded. Previously it bailed with already installed, leaving users stuck on the buggy hook after a repowise upgrade.
• re.sub backslash-escape fix. Passing the hook script as a string repl silently turned literal \n (inside a printf format) into real newlines, breaking shell quoting. The replacement now uses a callable repl to bypass escape processing.

Cross-platform notes

Git always runs hooks under POSIX sh (/bin/sh on Linux/macOS, git-bash on Windows), so the same script body is correct on all three platforms — no platform detection needed. The hook stays POSIX-clean (no [[ ]], no <( ), no arrays), verified by a sanity test.

Test plan

• tests/unit/cli/test_helpers.py — queued/pending marker round-trips, log rotation
• tests/unit/cli/test_hooks_legacy_migration.py — legacy strip, in-place upgrade, surrounding-hook preservation, POSIX-clean sanity check
• tests/unit/cli/test_augment_staleness.py — in-flight notice for lock and queued marker, fall-through to stale warning when markers are old, per-HEAD dedupe still works
• Manual end-to-end: fresh install, upgrade install over old marker body, hook fires correctly, queued marker written, log file populated, single-flight kicks in on rapid commits