feat(mcp): brain_read + brain_edit, contained to the sync source (brain#4) (#96)

brain_read returns a note verbatim; brain_edit is str_replace with an
exactly-once match, operating on the tree brain-sync commits from.

An adversarial design review before implementation changed three things:

1. Governance files are not in the model-facing tool. Brain content is
   ingested from the outside world and reaches a model's context, and that
   model would hold the editor — one poisoned page could rewrite the
   CLAUDE.md a .dispatch-allow agent later loads as instructions. And
   brain-sync merges with --strategy-option=ours, so the live tree beats a
   human's corrective push on the conflicting lines. CLAUDE.md now goes
   through brain_edit_root: de-surfaced from tools/list and gated on the
   local static principal. The scopes are disjoint — the privileged door
   has the smaller reach.

2. Reads needed the same allowlist, and brain_get already lacked it: it
   returned verbatim session transcripts under archive/ by path. A test
   proved the exposure live before the fix.

3. Canonicalise before allowlisting — knowledge/../.git/hooks/post-commit
   would otherwise reach a hook that brain-sync executes in a pod holding
   the Gitea push credentials.

Plus overlapping match counting, UTF-8 validation both ways, refusal while
git is mid-operation, flock with the read inside the lock, atomic
same-dir rename, and reindex only when frontmatter is touched.

Refs brain#4.

Co-authored-by: Mathias <mthbqv@gmail.com>

.dispatch-allow

@@ -0,0 +1,2 @@
+# Presence of this file marks this repo as opt-in for headless dispatch.
+# See dispatch#3.