fix(channel): roll back history on PromptCancelled to prevent poisoned turns

[Marenz]

The previous rollback fix (PR #104) correctly handles hard errors but misses PromptCancelled. This causes channels to permanently break after any cancelled turn (e.g. the reply tool fires and the hook terminates the turn).

Root cause: When Rig raises PromptCancelled, the history it carries was snapshotted before the tool batch executed. It contains the assistant's tool-call message but not the tool results. Writing that back leaves a dangling tool-call that poisons every subsequent turn with:

tool call result does not follow tool call (2013)

Concrete trace: reply tool fires → hook returns Terminate { "reply delivered" } → PromptCancelled raised → carried history has assistant(tool_call: reply) but no tool result → written to self.state.history → next turn fails permanently.

Fix: extract the history write-back into apply_history_after_turn and treat PromptCancelled like hard errors — truncate to history_len_before. The side-effects (reply sent, worker spawned, etc.) have already happened; we don't need the history to reflect them.

MaxTurnsError continues to write back — Rig pushes all tool results into a User message before raising it, so history is consistent.

Tests: five unit tests covering all four match arms plus an end-to-end scenario verifying the next turn is clean after a PromptCancelled rollback.

[jamiepine]

Perfect