feat: add native ACP subagent sessions

[nikita-ashihmin]

Summary

• negotiate the ACP subagents capability
• expose Codex collaborators as independent child session streams
• route nested messages, tools, approvals, and elicitations to the correct child
• emit exactly-once terminal lifecycle updates and keep the parent prompt open until announced children finish
• preserve legacy tool-call representation for clients without the capability

Compatibility

The published ACP SDK does not yet contain PR #1992 types, so the change isolates the temporary structural types in src/subagents/AcpSubagents.ts. Codex aggregate app-server notifications currently omit collaborator nickname/role; the adapter therefore uses a stable synthetic display name until that transport exposes them.

Async task scope

Codex app-server V2 currently exposes no independent background-task instance lifecycle with a stable task ID, progress, and terminal state. Legacy task_started / task_complete events represent the foreground turn, command executions remain ACP tool calls, and scheduled-task summaries describe plugin configuration rather than running work. This PR therefore does not advertise or synthesize AIR asyncTasks; support can be added when app-server exposes a real task lifecycle.

Validation

• npm run typecheck
• npm run build
• npm test (447 passed, 28 skipped)