chore(brain): de-surface brain_index from the model-facing toolset (#78)

brain_index is auto-called after every write, and its own description
said so. A verb surfaced to callers that callers should never call is
cognitive load with no decision behind it — ADR-0012 rule 4.

Removed from tools/list only. The dispatch case stays, so maintenance
can still rebuild a Map of Content by hand: de-surfacing hides the verb,
not the operation. A test pins both halves, since removing the descriptor
and removing the capability look identical from the tool list.

Nothing called it directly — grepped the skills, specs and docs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BeAp5LTscnz2W7Ubt4eJ6m

ingestion/internal/mcp/server.go

@@ -1,9 +1,14 @@
 // Package mcp implements an MCP HTTP handler for the ingestion service.
 // Exposed tools: brain_query, brain_write, brain_update, brain_get,
-// brain_pending, brain_promote, brain_index, brain_tunnel, brain_ingest,
+// brain_pending, brain_promote, brain_tunnel, brain_ingest,
 // brain_ingest_raw, brain_answer, brain_classify, brain_graph,
 // brain_context, session_log, and capture (the #55 relay tool, registered
 // only when WithCapture is set).
+//
+// brain_index dispatches but is deliberately absent from tools/list: it is
+// auto-called after every write, so surfacing it as a caller verb is
+// cognitive load with no decision behind it (ADR-0012 rule 4, #78). It stays
+// callable for maintenance — de-surfacing hides the verb, not the operation.
 package mcp
 
 import (