Both walk-level errors and ReadFile failures now use best-effort
semantics (warn via slog, continue) instead of mixed abort/silent-skip.
filepath.Rel error is now propagated from the callback instead of
discarded.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Implements search.Query which walks brainDir/wiki/**/*.md, scores files
by term-frequency across query tokens, and returns results sorted by
score descending. Uses only stdlib — no external search deps.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>