Review iteration: warn on gitSha read failure, WAL invariant comment, C6.2 wave-3 coordination gate

- backup.ts: log.warn when `git rev-parse HEAD` fails so sandbox/CI runs
  with no git show diagnosably instead of silently emitting gitSha:null
  (Karpathy review ask).
- backup.ts: expand comment on VACUUM INTO to pin the WAL-mode invariant
  — future readers must not "helpfully" add a wal_checkpoint call
  (Karpathy review ask).
- C6.2 proposal: add hard coordination gate before wave 3 because
  commit 0db6e42 already demonstrated the race hazard when two agents
  touch scripts/ / connector-definitions/ in the same window
  (Linus review ask).

docs/proposals/C6.2-error-taxonomy-phase2.md

@@ -93,6 +93,23 @@ Depends on Wave 1 only insofar as `registerHandle` wrapper lives in
 
 ### Wave 3 — database + credentials + browser + leftovers (≈58 sites)
 
+**⚠️ Coordination gate (hard)**: Wave 3 touches `src/main/database/**`
+(Linus's domain — backup hook, migrations, state-machine work) and
+`src/main/connector-definitions/**` (which was rehomed into a subdirectory
+during C6.1). Before starting Wave 3 the executor MUST:
+1. Check `git log --oneline -20` for any pending database / backup /
+   migration commits from other streams.
+2. Confirm B2 (tasks state-machine refactor) is either landed or
+   pre-gated — migrating database catches while B2 is mid-flight
+   produces diff storms that lose both sides' intent.
+3. Open a short coordination thread (issue or chat) with the database
+   owner before the first file edit — this wave is the highest-risk of
+   the three for scope conflicts.
+
+Skipping this gate previously caused commit 093718d in the C6.1 wave to
+accidentally carry an M4 scripts/ reorg because two agents touched
+`scripts/` in the same window. Do not repeat.
+
 Buckets:
 
 | Module | Approx sites | Dominant kind | Notes |