fix(mcp): drop the containment gate, always validate the post-edit envelope

Round-2 review found a third bypass: checkClaimEnvelope required old_str to
be fully contained within the extracted frontmatter block (which excludes
both "---" fence lines). An old_str disambiguated across the closing fence
into the body — a natural choice, since notes.Edit requires old_str to be
unique — failed that containment test, so the gate skipped validation
entirely while notes.Edit applied the replace verbatim, reaching the same
invalid-enum outcome as the two bypasses already closed.

Remove the containment gate structurally instead of patching another edge:
checkClaimEnvelope now always computes the post-edit doc and validates its
resulting frontmatter, regardless of old_str's textual relationship to the
fence. A body-only edit leaves the frontmatter unchanged (still valid, since
the note started valid), so this is a no-op for the common case and
fail-closed for every case that touches the envelope, however old_str is
phrased.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ingestion/internal/mcp/tools_notes.go

@@ -275,10 +275,15 @@ func checkClaimEnvelope(brainDir, path, oldStr, newStr string) error {
 		// — this function's job is the envelope, not read failures.
 		return nil
 	}
-	block, ok := frontmatterBlock(string(before))
-	if !ok || !strings.Contains(block, oldStr) {
-		return nil // old_str isn't in the frontmatter block at all
-	}
+	// No gate on where old_str falls relative to the frontmatter fence:
+	// round 2 found that requiring old_str to be fully contained in the
+	// extracted block was itself bypassable by an old_str disambiguated
+	// across the closing "---" into the body. Simpler and bypass-proof:
+	// always compute what the edit would produce and validate that
+	// resulting state's envelope. A body-only edit leaves the frontmatter
+	// (and thus the validation outcome) unchanged, so this is a no-op for
+	// the common case and fail-closed for every case that touches the
+	// envelope, regardless of how old_str is phrased.
 	after := strings.Replace(string(before), oldStr, newStr, 1)
 	return claim.ValidateEnvelope(frontmatterScalars(after))
 }