interrupt_thread: stop a thread's running turn #95

What: A new thread tool, interrupt_thread, lets an agent stop the running turn of a thread it can reach.
Why: Without it, a dispatcher that replaces a stalled or limit-hit worker can end up with two writers on one branch; only prism_cancel (runner-only) can stop a turn today.
So what: A worker implements it; the planner reviews and merges.

Parent

#92

Outcome

interrupt_thread(threadId) interrupts the target's active turn and returns once T3 reports it settled; an idle thread returns unchanged.

Elon record

• Requirements and who asked: Prism routes delegated work through Chromeria's thread tools; retire the Python runner #92 acceptance: replace a stalled child without a second writer. The runner interrupted stale workers for the same reason.
• Deleted: prism_cancel (removed in the hooks sub-Issue).
• Bottleneck: safe respawn after a stall.
• Checked myself: Chromeria's thread toolkit has no interrupt or cancel tool; upstream orchestration V2 has t3_thread_interrupt with the same behavior.

Acceptance criteria

• Interrupting a running child stops its turn and reports the settled state.
• Interrupting an idle or finished thread is a no-op with a clear result.
• Tool description says when to use it (before replacing a stalled or limit-hit child).

Non-goals

• Automatic interruption policies.

Blocked by

None

Required proof

• Tests at the threads toolkit seam; CI green on the exact head; independent review.