fix(tunnel): DetectTunnels walks and reads through brainfs (#174, #157 slice 5b)

DetectTunnels runs inside every brain_write and brain_update
(wikiUpkeep -> AutoTunnel). It walked wiki/ with filepath.WalkDir and
opened each note with a blocking os.Open: a FIFO named *.md anywhere
under wiki/ hung the write with no deadline, and a symlinked note's
title became a tunnel candidate (171k leaks in the swap race).

It now walks with brainfs.WalkRegular (regular files only) and reads
titles with readTitleAndCreatedIn (ReadRegular: O_NONBLOCK, handle and
SameFile checks). A wiki/ that is not a real directory yields no
candidates, as filepath.WalkDir did; other walk errors still abort. The
path-based readTitleAndCreated, now unused, is removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/brain/detect_root_test.go

@@ -0,0 +1,79 @@
+package brain_test
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs/brainfstest"
+)
+
+// hyperguild#174 / #157 slice 5b: DetectTunnels walked wiki/ and opened
+// every note with a blocking os.Open, so a FIFO named *.md anywhere under
+// wiki/ hung every brain_write and brain_update (wikiUpkeep -> AutoTunnel),
+// and a symlinked note's title became a tunnel candidate.
+func TestDetectTunnelsReturnsPromptlyWithAFIFOUnderWiki(t *testing.T) {
+	b := t.TempDir()
+	facts := filepath.Join(b, "wiki", "w", "facts")
+	require.NoError(t, os.MkdirAll(facts, 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(facts, "ok.md"), []byte("---\ntitle: Zqinside Phrase\n---\n"), 0o644))
+	require.NoError(t, syscall.Mkfifo(filepath.Join(facts, "f.md"), 0o644))
+	done := make(chan []brain.TunnelCandidate, 1)
+	go func() {
+		cs, _ := brain.DetectTunnels(b, "text mentioning zqinside phrase here")
+		done <- cs
+	}()
+	select {
+	case cs := <-done:
+		assert.Contains(t, fmt.Sprintf("%+v", cs), "Zqinside Phrase", "positive control")
+	case <-time.After(3 * time.Second):
+		if r, err := os.OpenFile(filepath.Join(facts, "f.md"), os.O_WRONLY|syscall.O_NONBLOCK, 0); err == nil {
+			_ = r.Close()
+		}
+		t.Fatal("DetectTunnels blocked on a FIFO")
+	}
+}
+
+func TestDetectTunnelsRefusesSymlinkedNotesAndSurvivesSwapRaces(t *testing.T) {
+	b := t.TempDir()
+	facts := filepath.Join(b, "wiki", "w", "facts")
+	require.NoError(t, os.MkdirAll(facts, 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(facts, "ok.md"), []byte("---\ntitle: Zqinside Phrase\n---\n"), 0o644))
+	require.NoError(t, os.Symlink(brainfstest.Outside(t), filepath.Join(facts, "out.md")))
+	inBrain := brainfstest.WriteSentinelAt(t, filepath.Join(b, "raw", "s.md"))
+	require.NoError(t, os.Symlink(filepath.Join("..", "..", "..", "raw", "s.md"), filepath.Join(facts, "in.md")))
+	content := "zqinside phrase and " + brainfstest.Sentinel
+
+	cs, err := brain.DetectTunnels(b, content)
+	require.NoError(t, err)
+	got := fmt.Sprintf("%+v", cs)
+	assert.Contains(t, got, "Zqinside Phrase", "positive control")
+	assert.NotContains(t, got, brainfstest.Sentinel)
+
+	p := filepath.Join(facts, "r.md")
+	read := func() string {
+		cs, _ := brain.DetectTunnels(b, content)
+		return fmt.Sprintf("%+v", cs)
+	}
+	brainfstest.AssertNoLeak(t, p, "---\ntitle: Zqinside Phrase\n---\n", 1500*time.Millisecond, read)
+	brainfstest.AssertNoLeakFrom(t, p, "---\ntitle: Zqinside Phrase\n---\n", inBrain, 1500*time.Millisecond, read)
+}
+
+// A wiki/ that is not a real directory yields no candidates and no error:
+// AutoTunnel runs inside brain_write.
+func TestDetectTunnelsSkipsAWikiThatIsNotARealDirectory(t *testing.T) {
+	b := t.TempDir()
+	require.NoError(t, os.MkdirAll(filepath.Join(b, "raw", "w", "facts"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(b, "raw", "w", "facts", "r.md"), []byte("---\ntitle: Zqrawonly Phrase\n---\n"), 0o644))
+	require.NoError(t, os.Symlink("raw", filepath.Join(b, "wiki")))
+	cs, err := brain.DetectTunnels(b, "zqrawonly phrase")
+	require.NoError(t, err)
+	assert.Empty(t, cs)
+}