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 cast in src/acp-subagents.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.

Validation

• npm run typecheck
• npm run build
• npm test (444 passed, 28 skipped)