fix(linkcheck): keep the write when the link index cannot be built (#178)

StripUnresolvedLinks runs before every brain_write and brain_update. Any
walk error below a linked tree's root (e.g. one unreadable subdirectory)
made it return an error, and the write was refused: the learning was
lost. Already true on main.

It now logs a warning and returns the content unchanged: a partial index
cannot tell an invented link from one into the unreadable part, so it
unwraps nothing (never inventing a resolution) and the note is written
with its links as given.

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

ingestion/internal/brain/linkcheck.go

@@ -47,7 +47,12 @@ var (
 func StripUnresolvedLinks(brainDir, content string) (string, []string, error) {
 	index, err := entryIndex(brainDir)
 	if err != nil {
-		return "", nil, err
+		// A partial index cannot tell an invented link from one into the
+		// part it could not read, so unwrap nothing, and do not refuse the
+		// write either: this runs before every brain_write, and a refused
+		// write is a lost learning (hyperguild#178).
+		slog.Warn("link check skipped: cannot index the brain", "err", err)
+		return content, nil, nil
 	}
 	if len(index) == 0 {
 		return content, nil, nil