New 'Projects' thread recap classification, gated off

What

• The background turn-state classifier gained a projectsRecap mode: when invoked with projectsRecap: true, it skips the normal preclassify/heuristic path, uses a distinct (server-overridable) system prompt, and asks the model to produce a recap "card" with a state of needs_reply, needs_approval, done, failed, or working, plus a "happened" summary. It retries once if the response JSON is invalid.
• The mid-turn/end-of-turn background-agent status classifier now passes a projectsRecap flag into the classification call that produces a task's working/idle/blocked detail text.
• A project's task/thread status can now use this richer "recap" object (headline, needs, needsYou, happened, reply) instead of the old flat detail/needs/output.result fields.
• Results and failures are logged via a projects_thread_recap telemetry event, with reasons like heuristic_engine, line_truncated, parse_failed, and side_query_failed.
• The whole feature is behind the tengu_projects_thread_recap flag and currently off.

Why This lays the groundwork for showing a more useful, model-written summary of what happened in a Projects background task, rather than a flat status line, though it isn't enabled yet.