fix(update,session): brain_update and session_log confine caller paths (#169)

Review round 1 on PR #170 found two more handlers with the #169 class of
bug (verified by execution):

- brain_update resolved its caller path with resolveWithin, a lexical
  containment check: any brain file could be superseded
  (classification.yaml, raw/, sessions/), and a final symlink was
  written through. UpdateNote now resolves like brain_edit
  (notes.Resolve, ScopeNotes: knowledge/ and wiki/ .md notes), and reads
  and rewrites through brainfs (ReadRegular, WriteRegular).
- session_log joined the caller's session_id onto the sessions dir and
  opened it O_CREATE|O_APPEND: an id with path segments created or
  appended a .jsonl file anywhere the pod could write. The id must now
  be a plain name (^[A-Za-z0-9_][A-Za-z0-9._-]{0,127}$, which every live
  session file matches), and the open adds O_NOFOLLOW|O_NONBLOCK. Read
  checks the id too.
- Tunnel tests: the .txt case now names an existing file (it failed on
  "missing" before), the symlink test adds an in-brain target, and the
  FIFO comment no longer claims the handle check is the only guard.

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

ingestion/internal/brain/tunnel_root_test.go

@@ -38,7 +38,7 @@ func TestWriteTunnelRefusesATargetThatTraversesOutOfTheBrain(t *testing.T) {
 
 func TestWriteTunnelRefusesUncleanOrNonWikiPaths(t *testing.T) {
 	brain0 := t.TempDir()
-	for _, p := range []string{"wiki/a/facts/s.md", "wiki/b/facts/t.md", "raw/r.md"} {
+	for _, p := range []string{"wiki/a/facts/s.md", "wiki/b/facts/t.md", "wiki/b/facts/t.txt", "raw/r.md"} {
 		require.NoError(t, os.MkdirAll(filepath.Dir(filepath.Join(brain0, p)), 0o755))
 		require.NoError(t, os.WriteFile(filepath.Join(brain0, p), []byte("x\n"), 0o644))
 	}
@@ -52,11 +52,13 @@ func TestWriteTunnelRefusesUncleanOrNonWikiPaths(t *testing.T) {
 	} {
 		assert.Error(t, brain.WriteTunnel(brain0, "wiki/a/facts/s.md", target), target)
 	}
-	got, err := os.ReadFile(filepath.Join(brain0, "raw", "r.md"))
-	require.NoError(t, err)
-	assert.Equal(t, "x\n", string(got))
+	for _, p := range []string{"raw/r.md", "wiki/b/facts/t.txt"} {
+		got, err := os.ReadFile(filepath.Join(brain0, p))
+		require.NoError(t, err)
+		assert.Equal(t, "x\n", string(got), p)
+	}
 	require.NoError(t, brain.WriteTunnel(brain0, "wiki/a/facts/s.md", "wiki/b/facts/t.md"), "positive control")
-	got, err = os.ReadFile(filepath.Join(brain0, "wiki", "b", "facts", "t.md"))
+	got, err := os.ReadFile(filepath.Join(brain0, "wiki", "b", "facts", "t.md"))
 	require.NoError(t, err)
 	assert.Contains(t, string(got), "[[a/facts/s]]")
 }
@@ -75,6 +77,17 @@ func TestWriteTunnelDoesNotWriteThroughASymlinkedNote(t *testing.T) {
 	got, err := os.ReadFile(victim)
 	require.NoError(t, err)
 	assert.Equal(t, "untouched\n", string(got))
+
+	// The same with a link to a file elsewhere in the brain, which os.Root
+	// would follow.
+	inVictim := filepath.Join(brain0, "raw", "v.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(inVictim), 0o755))
+	require.NoError(t, os.WriteFile(inVictim, []byte("untouched\n"), 0o644))
+	require.NoError(t, os.Symlink(filepath.Join("..", "..", "..", "raw", "v.md"), filepath.Join(tdir, "u.md")))
+	assert.Error(t, brain.WriteTunnel(brain0, "wiki/a/facts/s.md", "wiki/b/facts/u.md"))
+	got, err = os.ReadFile(inVictim)
+	require.NoError(t, err)
+	assert.Equal(t, "untouched\n", string(got))
 	got, err = os.ReadFile(src)
 	require.NoError(t, err)
 	assert.Equal(t, "x\n", string(got), "the source is not touched when the target is refused")