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/handlers.go

@@ -105,13 +105,6 @@ func (s *Server) tools() []map[string]any {
 				"target": str("path of target note (must be in a different wing)"),
 			}),
 		},
-		{
-			"name":        "brain_index",
-			"description": "Regenerate _index.md (Map of Content) for one or all wings under brain/wiki/. Auto-called after brain_write with wing+hall.",
-			"inputSchema": schema([]string{}, map[string]any{
-				"wing": str("optional wing to index; if absent, rebuilds every wing"),
-			}),
-		},
 		{
 			"name":        "brain_ingest_raw",
 			"description": "Ingest pre-structured pages into the brain wiki, bypassing the LLM extraction step.",