cancel_thread: new mcp tool — unify reminder + question cancel on both surfaces

TODO.md

@@ -40,13 +40,7 @@ how often the friction bites in normal use.
   into the prompt builder in `hive-ag3nt::turn.rs`. Even better: add a
   one-shot `recv_batch(max: u32)` MCP tool that returns up to `max`
   pending messages in a single round-trip.
-- **Self-management of own asks + reminders** — once I fire `ask` or
-  `remind` I have no way to inspect or cancel them from the agent side.
-  Operator can cancel asks via dashboard; nothing for reminders at all
-  (TODO above). Want `list_my_asks() -> [{id, target, question, asked_at}]`
-  and `cancel_ask(id)` on the agent surface, plus `list_my_reminders()`
-  / `cancel_reminder(id)`. Bounded by `asker == self` and `reminder.owner
-  == self` so no cross-agent meddling.
+- ~~**Self-management of own asks + reminders**~~ ✓ landed — unified with `get_open_threads` rather than a separate listing surface. `OpenThread` gained a `Reminder { id, owner, message, due_at, age_seconds }` variant (sub-agent flavour filters by `owner == self`; manager unfiltered). New `mcp__hyperhive__cancel_thread(kind, id)` on both surfaces — `kind` is `"question"` (asker gets `[cancelled by <self>]` answer, unblocks) or `"reminder"` (hard-deleted before fire). Auth: sub-agent must own the row; manager bypasses for hive-wide cleanup. New helpers `OperatorQuestions::cancel` + `Broker::cancel_reminder_as` push the auth check down so both flavours stay aligned. Shared dispatch in `hive-c0re/src/questions.rs::handle_cancel_thread`.
 - **Optional `in_reply_to: <msg_id>` on send** — pure wire addition; no
   behavioural change. The dashboard could render conversation threads
   (already wants this for the agent-to-agent question UI in the