agency: auto-default --spawn-topic by thread context

[MagMueller]

Auto-default the spawn-topic flag based on where `agency-report` is invoked from. Magnus's exact rule, encoded as code (not just guidance) so the agent doesn't have to remember.

Rule

• Posting from a thread that's already a `worker_topic` for some prior card → default `--no-spawn-topic` (in-place). The agent is already deep in one task; follow-up cards keep the conversation in the same thread.
• Posting from anywhere else (the main agency feed, a fresh chat, the /agency loop) → default `--spawn-topic` (each card gets its own forum topic).

Implementation

• `agency_db.is_worker_topic(thread_id) → bool` — True iff the thread was spawned as worker_topic for some other suggestion. Excludes in-place bookkeeping (cards whose own `tg_thread_id == worker_topic_id`).
• `agency-report` flag becomes tristate via mutually-exclusive group + `store_const`:
  • `--spawn-topic` → force True
  • `--no-spawn-topic` → force False
  • neither → `args.spawn_topic is None` → resolved via `is_worker_topic(args.thread_id)`
• Existing downstream code (default-button labels, deep-link glue) reads the resolved `args.spawn_topic` and continues to work unchanged.

Test plan

• Post from main agency topic (not a worker_topic) without flags → card shows `🧵 Yes (new thread)` labels (auto-defaulted to spawn)
• Tap Yes → fresh forum topic spawns
• Post a follow-up card from inside that worker topic without flags → card shows `✅ Yes` labels (auto-defaulted to in-place)
• Tap Yes on the follow-up → dispatch runs in the same worker topic, no nested topic
• Force-spawn from inside a worker topic via `--spawn-topic` → fresh topic spawns anyway (override honored)
• Force-in-place from main feed via `--no-spawn-topic` → runs in main feed thread (override honored)

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Summary by cubic

Auto-defaults --spawn-topic in agency-report based on thread context. Posts inside an existing worker topic run in-place by default; posts from other threads spawn a new topic.

• New Features
  • Added agency_db.is_worker_topic(thread_id) to detect worker topics (excludes in-place bookkeeping).
  • Made spawn behavior tri-state: --spawn-topic (force spawn), --no-spawn-topic (force in-place), or no flag (auto-detect).
  • Explicit flags override the auto-default.
  • Button labels and deep-linking keep working off the resolved args.spawn_topic.

^{Written for commit c7a0d6b. Summary will update on new commits.}