Claude, Codex, OpenCode, and ACP each reimplemented the same `run()`
orchestration: pre-start event buffering, turn-id filtering, terminal
event settling, timeline collection, and final-text reduction. Hoist
the loop into `provider-runner.ts` with a small adapter (startTurn,
subscribe, getSessionId) and pluggable final-text reducer.
Net -252 lines across the four providers. Also fixes a latent bug
where a `turn_failed` emitted before startTurn returned was buffered
and silently swallowed instead of rejecting the run.
