feat(acp): support Lody subagent events and automatic session titles

[frostming]

Summary

Implement two Lody extension contracts from acp-extension-core, both gated on the client identifying itself as Lody in InitializeRequest.clientInfo.

Subagent events

• Add bub_acp_server.subagents with the _lody/subagents/event wire contract: capability negotiation, a per-execution SubagentEventEmitter, JSON-safe payloads, and best-effort delivery that never fails the run.
• Bub's subagent tool consumes the child stream internally, so replace the runtime agent's tool with a streaming variant when the client negotiates the capability. The replacement keeps the original name, description, parameters, renderer and output schema, and still returns Bub's result, so the model sees no difference and the global registry stays untouched.
• Map child stream events onto snapshot, sparse progress and output payloads. outputRead is none and cancel is false, since runs are not addressable through the separate list/cancel/output request surface.
• Negotiation stays bilateral: a client opt-in is honored even when the agent did not advertise, as the contract requires.

Session titles

• Advertise sessionTitle and push a tagged session_info_update after a session's first turn, then persist the title for session/list.
• Generate with one lean completion: a throwaway temp/ session with no tools and no skills. Best effort, bounded by a timeout, at most once per session, skipped for sessions that already have a title.

Capabilities

• Report agentCapabilities._meta.lody only to Lody clients; other clients get no Lody-namespaced capability and no Lody-specific behavior.
• Keep the non-Lody {"steering": {"supported": true}} field unconditional so clients using the original Codex steering route can still detect it.

Testing

• Tests cover the wire contract, negotiation, failure and cancellation paths, client identification, and a real stdio end-to-end run.
• New test files: test_subagents.py, test_session_title.py.

Files changed

• packages/bub-acp-server/README.md
• packages/bub-acp-server/src/bub_acp_server/agent.py
• packages/bub-acp-server/src/bub_acp_server/subagents.py (new)
• packages/bub-acp-server/tests/test_e2e.py
• packages/bub-acp-server/tests/test_mcp.py
• packages/bub-acp-server/tests/test_plugin.py
• packages/bub-acp-server/tests/test_session_title.py (new)
• packages/bub-acp-server/tests/test_subagents.py (new)