feat(daemon): wire agent.run/stop/send/attach RPCs with process spawn + event stream

[Codename-11]

Summary

• Implements AgentRuntime in packages/daemon/src/agent-runtime.ts that spawns child processes, tracks status (starting → running → completed/failed/stopped), publishes events via the Hub on topic agent:<id>, and persists them to the agent_events table with (epoch, seq) cursor.
• Replaces stubbed agent.run / agent.stop / agent.send handlers with real runtime calls, and adds a new agent.attach RPC that returns persisted events (supports sinceEpoch / sinceSeq / limit for replay).
• Frames channel-1 terminal bytes with a 36-byte agent UUID prefix so the client can route multiplexed streams. Codec is shared via packages/daemon/src/terminal-frame.ts.
• Extends the client SDK with client.agents.attach(agentId, { onEvent, onTerminal }), per-agent terminal handler map, AgentAttachParams / AgentAttachResult / AgentEventPayload schemas, and Methods.agent_attach.
• Ships a minimal internal _echo profile that spawns a Node one-liner echoing stdin → stdout so tests + smoke scripts can exercise the pipe without a real adapter wired yet.

Test plan

• npx tsc --noEmit passes for both packages/daemon and packages/client (pre-existing rootDir noise from @axiom-labs/arc-core imports is unrelated to this change).
• npx vitest run packages/daemon/tests/agent-lifecycle.test.ts packages/daemon/tests/daemon.test.ts — 7/7 pass:
  • runs, streams terminal bytes, persists events, and exits cleanly
  • agent.attach replays persisted events after exit
  • agent.stop terminates a long-running process
  • plus the 4 pre-existing daemon/client tests.
• E2E smoke: started daemon in tmp ARC_DIR, spawned _echo agent via agent.run({ profile: "_echo", prompt: "hello" }), subscribed via agent.attach, waited for exit event, confirmed terminal bytes carry the echoed prompt and final DB status is completed.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

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