v1.53.0 — Obsidian read-back recall (bidirectional)

The Obsidian bridge is now bidirectional. CDT syncs its vault into Obsidian and reads back: at task start cdt-recall ranks your Obsidian vault (your curated notes + synced CDT content) with a dependency-free BM25 ranker and surfaces the most relevant notes alongside the vault's own lessons — so Obsidian knowledge actively informs CDT's work. New cdt-obsidian recall <query> (self-gating, fail-open) + cdt-config obsidian-recall-root <path>. Pure stdlib, no embeddings. TDD'd + wired into CI. Full notes in CHANGELOG.