feat(acp): dispose each session's agent on disconnect/teardown

The bridge now holds each session's `AgentHandle` disposer in its
`SessionRecord` and runs it on teardown (client disconnect or fiber dispose)
instead of the old `abort()` + `whenIdle()` drain that left agents
registered. A bare client disconnect now leaves NO registered agent and NO
session-store entry — not an idled-but-still-registered one. The queue-aware
`cancel()` inside the disposer also closes the former pre-step best-effort
window (a turn about to start is dropped), so teardown reaches true
quiescence.

The `session/load`-races-teardown leak is fixed: if the bridge closed while
`resume()` was pending, the just-resumed handle is disposed before throwing,
so it leaves no orphan (it has no SessionRecord, so quiesce() never sees it).

Tests: the disconnect test now asserts (through the SAME memoized teardown)
that the agent is unregistered AND its session removed; a durability test
re-loads the persisted log after dispose and asserts the closing turn/end is
on disk (guards the teardown-order contract); a sibling-isolation test proves
one handle's dispose() leaves other agents untouched. Docs: agent /
agent-loop / acp READMEs, architecture.md, and the stale in-code quiesce()
ownership comment updated to the per-agent disposal model; the now-resolved
TODO(rfc010-agent-disposal) / TODO(rfc010-cancel-prestep) teardown notes
removed.

examples/coding-agent/tests/resume.e2e.ts

@@ -41,7 +41,7 @@ describe.skipIf(!process.env.DEEPSEEK_API_KEY)('resume: continue a persisted ses
       agentId: 'resume-1',
       sessionId: SESSION_ID,
       agentOptions: { model: 'deepseek-v4-flash', systemPrompt: SYSTEM_PROMPT },
-    }) as ReactLoopAgent
+    }).agent as ReactLoopAgent
     first.send([{ type: 'text', text: `Remember this code for later: ${SECRET}. Just acknowledge it.` }])
     await waitForIdle(ctx, first)
     await ctx.fiber.dispose()
@@ -51,11 +51,11 @@ describe.skipIf(!process.env.DEEPSEEK_API_KEY)('resume: continue a persisted ses
     // session. The loaded event log seeds the live session, so the model sees
     // run 1's exchange as conversation history.
     ctx = await codingHarness(process.cwd(), root)
-    const resumed = await ctx.agents.resume({
+    const resumed = (await ctx.agents.resume({
       agentId: 'resume-2',
       resumeSessionId: SESSION_ID,
       agentOptions: { model: 'deepseek-v4-flash', systemPrompt: SYSTEM_PROMPT },
-    }) as ReactLoopAgent
+    })).agent as ReactLoopAgent
     expect(resumed.session.id).toBe(SESSION_ID)
     // The prior user turn is in the rehydrated log before the model is asked.
     expect(JSON.stringify(resumed.session.deriveMessages())).toContain(SECRET)