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_test.go

@@ -122,6 +122,92 @@ func TestBrainQueryWingScope(t *testing.T) {
 	assert.NotContains(t, text, "wiki/other/facts/y.md")
 }
 
+func TestBrainWriteAutoTunnelsOnExactMatch(t *testing.T) {
+	brainDir := t.TempDir()
+	// Seed a pre-existing note in wing "other".
+	existing := filepath.Join(brainDir, "wiki/other/facts/widget.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(existing), 0o755))
+	require.NoError(t, os.WriteFile(existing,
+		[]byte("---\nwing: other\nhall: facts\ntitle: Widget\n---\nbody.\n"), 0o644))
+
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+
+	// Write a new note in a *different* wing whose content references "Widget".
+	resp := toolCall(t, srv, "brain_write", map[string]any{
+		"content":  "# Notes\n\nThis note discusses the Widget concept.\n",
+		"filename": "notes",
+		"wing":     "jepa-fx",
+		"hall":     "facts",
+	})
+	require.Nil(t, resp["error"])
+
+	newNote := filepath.Join(brainDir, "wiki/jepa-fx/facts/notes.md")
+	got, err := os.ReadFile(newNote)
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "[[other/facts/widget]]", "new note should link to existing")
+
+	gotTgt, err := os.ReadFile(existing)
+	require.NoError(t, err)
+	assert.Contains(t, string(gotTgt), "[[jepa-fx/facts/notes]]", "existing note should backlink")
+}
+
+func TestBrainWriteAutoTunnelSkipsSameWing(t *testing.T) {
+	brainDir := t.TempDir()
+	existing := filepath.Join(brainDir, "wiki/jepa-fx/facts/widget.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(existing), 0o755))
+	require.NoError(t, os.WriteFile(existing,
+		[]byte("---\nwing: jepa-fx\nhall: facts\ntitle: Widget\n---\nbody.\n"), 0o644))
+
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+	resp := toolCall(t, srv, "brain_write", map[string]any{
+		"content":  "Same wing reference to Widget here.\n",
+		"filename": "notes",
+		"wing":     "jepa-fx",
+		"hall":     "facts",
+	})
+	require.Nil(t, resp["error"])
+
+	newNote := filepath.Join(brainDir, "wiki/jepa-fx/facts/notes.md")
+	got, err := os.ReadFile(newNote)
+	require.NoError(t, err)
+	assert.NotContains(t, string(got), "[[jepa-fx/facts/widget]]", "same-wing match must not auto-tunnel")
+}
+
+func TestBrainTunnelLinksTwoNotes(t *testing.T) {
+	brainDir := t.TempDir()
+	for _, p := range []struct{ rel, body string }{
+		{"wiki/jepa-fx/decisions/val-vol.md", "---\nwing: jepa-fx\nhall: decisions\n---\n# Val Vol\n"},
+		{"wiki/hyperguild/decisions/routing.md", "---\nwing: hyperguild\nhall: decisions\n---\n# Routing\n"},
+	} {
+		full := filepath.Join(brainDir, p.rel)
+		require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+		require.NoError(t, os.WriteFile(full, []byte(p.body), 0o644))
+	}
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+	resp := toolCall(t, srv, "brain_tunnel", map[string]any{
+		"source": "wiki/jepa-fx/decisions/val-vol.md",
+		"target": "wiki/hyperguild/decisions/routing.md",
+	})
+	require.Nil(t, resp["error"])
+
+	src, err := os.ReadFile(filepath.Join(brainDir, "wiki/jepa-fx/decisions/val-vol.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(src), "[[hyperguild/decisions/routing]]")
+	tgt, err := os.ReadFile(filepath.Join(brainDir, "wiki/hyperguild/decisions/routing.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(tgt), "[[jepa-fx/decisions/val-vol]]")
+}
+
+func TestBrainTunnelRejectsMissing(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil, nil)
+	resp := toolCall(t, srv, "brain_tunnel", map[string]any{
+		"source": "wiki/a/facts/ghost.md",
+		"target": "wiki/b/facts/ghost.md",
+	})
+	require.NotNil(t, resp["error"])
+}
+
 func TestBrainWriteRejectsTraversal(t *testing.T) {
 	brainDir := t.TempDir()
 	srv := mcp.NewServer(brainDir, nil, nil, nil)