Vouch agent-initiated ACP turns so async job delivery renders

[bradhallett]

What was wrong

ACP agents can emit session/update work output with no prompt in flight — an agent-initiated turn (OMP's async-job auto-delivery is the concrete case: a background job settles, the agent resumes and streams a follow-up unprompted). ACP brackets no turn for that, and the bridge forwarded the updates unbracketed, so the translator classified them unhandled/onlyIfNoTurn and they never reached the thread: the agent's output was silently dropped. Root cause and evidence in #2122 (DB event gap, wire probe, pre-fix permalinks).

What changed

plugins/provider-acp/src/bridge/bridge.ts — the bridge now vouches agent-initiated turns itself, the shape docs/provider-bridge-protocol.md turn lifecycle rule 3 sanctions (and the compaction path already uses):

• Idle agent-work updates (agent_message_chunk, agent_thought_chunk, tool_call, tool_call_update, plan) open a vouched turn via the existing bridge turn-started command → turn.open.
• A quiet window (SPONTANEOUS_TURN_IDLE_TIMEOUT_MS, 120 s, re-armed per chunk) settles the turn as end_turn; a still-open one settles before the next bb-initiated turn or compaction starts; thread/stop and session teardown cancel/clean it. A slow provider can split into multiple vouched turns; output is never lost.
• Non-work updates (available_commands_update, usage_update, …) stay noise — no phantom turns. user_message_chunk stays classified as input-echo noise, so the replayed async result creates no phantom user row.

No wire/protocol changes; translator untouched (stays context-free). fake-acp-agent.mjs gains spontaneous-stream:N (post-prompt unsolicited user_message_chunk + N agent chunks) and spontaneous-noise behaviors plus a follow-up usage_update signal; __setSpontaneousTurnIdleTimeoutForTests shrinks the quiet window in tests.

How you verified

• 3 new tests in src/bridge/bridge.test.ts (real bridge + real delta assembler, fake agent over stdio): vouched turn opens for unsolicited output and quiet-closes; a still-open vouched turn settles before the next user turn (3 started / 3 completed, stream intact, exactly one input.accepted); non-work idle updates open nothing.
• Mutation check: removing agent_message_chunk from the work-kind map fails both positive tests (and not the noise test).
• pnpm -C plugins/provider-acp test → 184 passed (14 files); tsc --noEmit clean; eslint + prettier clean on touched files.

Fixes #2122

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