feat(recall): surface source-file anchors for codebase hits

[m0Nst3r873]

Summary

Resolves #233. teamwiki code-knowledge pages carry source: file paths in
their frontmatter, but recall discarded them — only the title was
regex-scraped. An agent doing an edit received architecture prose without
knowing which files a change touches, so it fell back to exploring the
codebase anyway (exactly the pain reported in #233).

This surfaces the source-file anchors and lets the recall subagent turn
them into an actionable "how to change X" starting point.

Changes

• code-knowledge-recall.ts: parse frontmatter via gray-matter; add
  sanitizeSources() to keep only in-repo file-path anchors (drop URLs and
  bare directories); thread sources through PageDoc → CodeKnowledgeResult.
• recall.ts: formatResults emits a Sources: line (before the
  snippet) for hits that carry anchors. Plain learnings/docs entries are
  unaffected.
• agents/teamai-recall.md: for edit/change queries, use --depth lookup
  and build a Change entry points section from graph-index.json forward
  edges (1–3 entry files, skip self-edges, cap 10) plus a suggested reading
  order. Derived solely from existing graph data — no invented relationships.
• docs: note the Sources: line in README (EN + zh-CN).

Test Plan

• npx tsc --noEmit — clean
• npx vitest run — 1773 passed (added source three-state, sanitize,
  extensionless-file, and formatResults Sources: cases)
• npm run build — OK
• Real CLI: teamai recall "..." --depth lookup surfaces file anchors
  (e.g. Sources: src/providers/github/gh-cli.ts, ...); --depth context
  no longer leaks the repo-URL noise (sanitized)
• End-to-end subagent simulation ("how to add a hook handler"): produces
  entry file + directed dependency chain + reading order

Known limitations (follow-up)

• Facts pages that carry the richest anchors are outside the context
  whitelist and rank low under BM25 for edit queries; this PR relies on the
  subagent's targeted graph-index.json read to bypass that. Fixing the
  retrieval ranking itself is deferred to a separate change.

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