Expose Codex subagent activity over ACP

[nikita-ashihmin]

Summary

• map Codex subAgentActivity items to standard ACP tool_call updates
• attach thread identity, agent path, and activity kind under namespaced _meta.codex.subagent metadata
• preserve collaboration model/reasoning metadata and add _meta.codex.collaboration
• replay the same subagent events from loaded session history

Rationale

Real Codex app-server runs report a spawn as a completed subAgentActivity item rather than a collabAgentToolCall(spawnAgent). The adapter currently ignores that item, so ACP clients only see subsequent wait collaboration calls. ACP v1 has no dedicated subagent update or tool kind, so this uses the spec-compatible tool_call(kind: other) representation and the reserved namespaced _meta extension point.

Child-thread messages and tools are intentionally not forwarded into the parent ACP session in this change. Doing that safely requires explicit client capability negotiation so clients that do not understand parent metadata do not render child events at the top level.

Verification

• npm run typecheck
• npm test (311 passed, 28 skipped)
• npm run build
• real Codex app-server run through npm run codex-test to verify the emitted subAgentActivity shape