feat(brain): cross-wing tunnels — bidirectional wikilinks + auto-detect

Adds the `brain_tunnel` MCP tool and auto-tunnel behaviour for
`brain_write`, so concepts that appear in multiple wings become
navigable from any of them.

New surface in package brain:
- WriteTunnel(brainDir, src, tgt) — appends a `## See also` bidirectional
  wikilink between two notes in different wings. Idempotent (link not
  duplicated on re-call) and reuses an existing See also section.
- DetectTunnels(brainDir, content) — walks brain/wiki/, returns
  TunnelCandidates for notes whose title appears in content. Tags
  whole-word case-insensitive hits as Exact=true and substring-only hits
  as Exact=false.
- AutoTunnel(brainDir, src, content) — wraps DetectTunnels: writes
  cross-wing exact matches, stages fuzzy matches into
  brain/raw/tunnel-candidates-<YYYY-MM-DD>.md for human review.

MCP wiring:
- `brain_tunnel` tool: explicit manual link (source, target).
- `brain_write` with wing+hall now triggers AutoTunnel on the new
  content. Failures are logged and never abort the primary write.

readTitleAndCreated also humanises the slug fallback (hyphens → spaces)
so titleless notes participate in content matching.

Closes hyperguild#16.

Tests: idempotency, same-wing rejection, missing-note rejection,
See-also reuse, exact/fuzzy detection, slug fallback, MCP tool happy
path, auto-tunnel hook (cross-wing exact → linked; same-wing → skipped;
fuzzy → candidates file).

ingestion/internal/mcp/server.go

@@ -1,6 +1,6 @@
 // Package mcp implements an MCP HTTP handler for the ingestion service.
-// Exposed tools: brain_query, brain_write, brain_index, brain_ingest,
-// brain_ingest_raw, brain_answer, brain_classify, session_log.
+// Exposed tools: brain_query, brain_write, brain_index, brain_tunnel,
+// brain_ingest, brain_ingest_raw, brain_answer, brain_classify, session_log.
 package mcp
 
 import (
@@ -139,6 +139,8 @@ func (s *Server) handleCall(ctx context.Context, name string, args json.RawMessa
 		return s.brainWrite(ctx, args)
 	case "brain_index":
 		return s.brainIndex(ctx, args)
+	case "brain_tunnel":
+		return s.brainTunnel(ctx, args)
 	case "brain_ingest_raw":
 		return s.brainIngestRaw(ctx, args)
 	case "brain_ingest":