fix(brainfs): bound the read in ReadRegularMax; honest test names (#157)

Review round 1 on PR #165:

- ReadRegularMax checked the size on the opened handle, then read with an
  unbounded io.ReadAll, so a file growing after the check returned past the
  limit. The read now goes through io.LimitReader(max+1). A test seam grows
  the file between the check and the read and counts bytes consumed.
- New: ReadScoped returns ErrTooLarge for a note over maxNoteBytes; red if
  readContained drops the limit.
- notes.Target carries the brain root instead of deriving it by TrimSuffix.
- Rule 9: reader tests renamed to <Reader>RefusesSymlinksAndSurvivesSwapRaces;
  ReadNote test has a positive control; brainfstest comment states what the
  harness observes and what it does not.

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

ingestion/internal/api/readnote_root_test.go

@@ -15,7 +15,7 @@ import (
 
 // hyperguild#157 slice 2: ReadNote (brain_get, read-after-write) read by
 // path; it now reads through brainfs.
-func TestReadNoteIsConfinedToRegularFilesInTheBrain(t *testing.T) {
+func TestReadNoteRefusesSymlinksAndSurvivesSwapRaces(t *testing.T) {
 	brain := t.TempDir()
 	facts := filepath.Join(brain, "wiki", "w", "facts")
 	require.NoError(t, os.MkdirAll(facts, 0o755))
@@ -25,6 +25,11 @@ func TestReadNoteIsConfinedToRegularFilesInTheBrain(t *testing.T) {
 	assert.NotContains(t, body, brainfstest.Sentinel)
 
 	p := filepath.Join(facts, "r.md")
+	require.NoError(t, os.WriteFile(p, []byte("---\nx: y\n---\ninside"), 0o644))
+	fm, body, _, err := api.ReadNote(brain, "wiki/w/facts/r.md")
+	require.NoError(t, err)
+	assert.Equal(t, "y", fm["x"])
+	assert.Contains(t, body, "inside")
 	read := func() string {
 		fm, body, _, _ := api.ReadNote(brain, "wiki/w/facts/r.md")
 		s := body