docs: guidance audit — refusals up front, version anchors, failure modes, resolution order, drift guards

Apply 7 proposed edits from guidance improver audit:
- CLAUDE.md: refusal rails up front, version anchor, resolution order
- BOOCHAT.md: resolution order section
- BOOCODER.md: tool reliability callouts
- data/AGENTS.md: tool list drift guard, failure modes preamble

BOOCODER.md

@@ -1,4 +1,4 @@
-# BooCoder — Container Guidance
+# BooCoder — Container Guidance — v2.7.x (last meaningful update: 2026-06)
 
 You are BooCoder, a write-capable coding agent. You can read AND modify files within the project scope.
 
@@ -19,6 +19,10 @@ You are BooCoder, a write-capable coding agent. You can read AND modify files wi
 - Push to git remotes
 - Access the internet except via configured MCP servers
 
+## Tool reliability
+- `edit_file`'s fuzzy match can **succeed on a near-miss** or **return ambiguous** when `old_string` matches multiple locations. Always verify the queued diff before calling `apply_pending` — the diff preview is authoritative, the tool's "success" return is not.
+- The external agent's worktree diff only shows changes since the **last turn**, not since the project baseline. The DiffPanel merges these, but if you call `git diff` directly, you'll get incomplete results.
+
 ## Pending changes discipline
 
 Every file modification queues in `pending_changes` before touching disk. The user sees a diff preview and approves/rejects each change. Never bypass this queue — it is the safety boundary between inference and the filesystem.