fix(linkcheck): skip a linked tree that is not a real directory (#157)

Review round 1 on PR #175:

- entryIndex returned WalkRegular's walk-root ErrNotRegular as an error,
  and StripUnresolvedLinks runs before every brain_write: a symlinked,
  dangling or regular-file knowledge/ (or wiki/) made every write fail
  and lose the learning. main's filepath.WalkDir skipped such a root. It
  is now skipped with a warning; other walk errors still abort, as on
  main. New table test over the three shapes.
- The symlinked-hall assertion passed with no index written at all. It
  now indexes a real hall next to the symlinked one and asserts no
  error, the index exists and lists the real note; a new test does the
  same for BuildAllWingIndexes with a symlinked wing next to a real one.
- The test comment no longer says this PR fixed the FIFO hang on every
  brain_write: that path is tunnel.DetectTunnels (slice 5b).

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

ingestion/internal/brain/linkcheck.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"io/fs"
+	"log/slog"
 	"path"
 	"regexp"
 	"sort"
@@ -121,6 +122,13 @@ func entryIndex(brainDir string) (map[string]struct{}, error) {
 			index[strings.TrimPrefix(strings.TrimSuffix(rel, ".md"), "wiki/")] = struct{}{}
 			return nil
 		}, func(rel string, err error) {
+			// A linked tree that is not a real directory (a symlink, a
+			// file) is skipped, as filepath.WalkDir did: this runs before
+			// every brain_write, and an error here loses the learning.
+			if rel == dir && errors.Is(err, brainfs.ErrNotRegular) {
+				slog.Warn("link check: skipping a linked tree that is not a real directory", "dir", dir)
+				return
+			}
 			if walkErr == nil {
 				walkErr = fmt.Errorf("%s: %w", rel, err)
 			}