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/handlers.go

@@ -60,6 +60,14 @@ func (s *Server) tools() []map[string]any {
 				"hall":     enum("optional memory type (requires wing)", halls...),
 			}),
 		},
+		{
+			"name":        "brain_tunnel",
+			"description": "Create an explicit bidirectional [[wikilink]] between two notes in different wings. Idempotent.",
+			"inputSchema": schema([]string{"source", "target"}, map[string]any{
+				"source": str("path of source note relative to brain dir, e.g. wiki/jepa-fx/decisions/val-vol.md"),
+				"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.",
@@ -174,16 +182,38 @@ func (s *Server) brainWrite(ctx context.Context, args json.RawMessage) (json.Raw
 		return nil, err
 	}
 	// Auto-regenerate the wing _index.md when the write landed in the
-	// structured wiki. A failure here is best-effort — log and move on,
-	// since the note itself is already written.
+	// structured wiki, and auto-tunnel cross-wing matches. Both are
+	// best-effort: the note is already written.
 	if a.Wing != "" && a.Hall != "" {
 		if err := brain.BuildWingIndex(s.brainDir, a.Wing); err != nil {
 			slog.Warn("brain_write: auto-index failed", "wing", a.Wing, "err", err)
 		}
+		if err := brain.AutoTunnel(s.brainDir, relPath, a.Content); err != nil {
+			slog.Warn("brain_write: auto-tunnel failed", "src", relPath, "err", err)
+		}
 	}
 	return json.Marshal(map[string]string{"path": relPath})
 }
 
+type brainTunnelArgs struct {
+	Source string `json:"source"`
+	Target string `json:"target"`
+}
+
+func (s *Server) brainTunnel(ctx context.Context, args json.RawMessage) (json.RawMessage, error) {
+	var a brainTunnelArgs
+	if err := json.Unmarshal(args, &a); err != nil {
+		return nil, fmt.Errorf("parse args: %w", err)
+	}
+	if a.Source == "" || a.Target == "" {
+		return nil, fmt.Errorf("source and target are required")
+	}
+	if err := brain.WriteTunnel(s.brainDir, a.Source, a.Target); err != nil {
+		return nil, fmt.Errorf("tunnel: %w", err)
+	}
+	return json.Marshal(map[string]string{"status": "ok"})
+}
+
 type brainIndexArgs struct {
 	Wing string `json:"wing,omitempty"`
 }