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/mcp/readexcerpt_root_test.go

@@ -15,7 +15,7 @@ import (
 // hyperguild#157 slice 2: readExcerpt (brain_context) read by path, so a
 // symlink in the Syncthing-shared brain put an outside file in context, and
 // a FIFO hung it. It now reads through brainfs.
-func TestReadExcerptIsConfinedToRegularFilesInTheBrain(t *testing.T) {
+func TestReadExcerptRefusesSymlinksAndSurvivesSwapRaces(t *testing.T) {
 	brain := t.TempDir()
 	facts := filepath.Join(brain, "wiki", "w", "facts")
 	require.NoError(t, os.MkdirAll(facts, 0o755))