fix(brainfs,session): clean paths only; session logs open through the brain root (#173)

Review round 1 on PR #187:

- realDir checked the cleaned parent while Lstat/OpenFile used the raw
  path; os.Root resolves ".." after following a symlink, so a ".." placed
  after a symlinked directory passed the check and redirected the read or
  write. ReadRegular, ReadRegularMax and WriteRegular now refuse a path
  that is not clean and local (ErrNotRegular). No current entry point
  reached it (callers clean first); the primitive no longer relies on that.
- session.Append checked the sessions dir with Lstat, then opened by plain
  path: a symlink swapped in between created logs outside the brain
  (29k in the reviewer's RENAME_EXCHANGE race). It now opens through an
  os.Root on the sessions dir's parent, so a swap can only redirect inside
  the brain (#167), and, since os.Root follows an in-root final symlink
  even with O_NOFOLLOW, writes only after confirming the path is the
  regular file it opened (Lstat + os.SameFile).
- Endpoint tests assert ErrorIs(brainfs.ErrNotRegular) and have positive
  controls.

golang.org/x/sys moves from indirect to direct (unix.Renameat2 in the
session race test); it was already in the module graph.

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

ingestion/internal/brain/tunnel_redirect_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
 )
 
 // hyperguild#173: brain_tunnel's write followed a directory symlink inside
@@ -24,9 +25,14 @@ func TestWriteTunnelRefusesATargetBehindASymlinkedDirectory(t *testing.T) {
 	require.NoError(t, os.MkdirAll(filepath.Join(b, "wiki", "b"), 0o755))
 	require.NoError(t, os.Symlink(filepath.Join("..", "..", "raw", "facts"), filepath.Join(b, "wiki", "b", "facts")))
 
-	assert.Error(t, brain.WriteTunnel(b, "wiki/a/facts/s.md", "wiki/b/facts/v.md"))
+	assert.ErrorIs(t, brain.WriteTunnel(b, "wiki/a/facts/s.md", "wiki/b/facts/v.md"), brainfs.ErrNotRegular)
 	got, _ := os.ReadFile(victim)
 	assert.Equal(t, "untouched\n", string(got))
 	got, _ = os.ReadFile(src)
 	assert.Equal(t, "x\n", string(got), "the source is not rewritten either")
+
+	real := filepath.Join(b, "wiki", "c", "facts", "t.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(real), 0o755))
+	require.NoError(t, os.WriteFile(real, []byte("t\n"), 0o644))
+	require.NoError(t, brain.WriteTunnel(b, "wiki/a/facts/s.md", "wiki/c/facts/t.md"), "positive control: a tunnel between real directories is written")
 }