chore: update ACP SDK to 1.8

[OldKrab]

Summary

Bumps @agentclientprotocol/sdk from ~1.5.0 to ~1.8.0 and adapts the v2 code to the regenerated schema types. The pin did not move with #620, and 1.6–1.8 each add type errors against current main (1, 4 and 6 respectively).

Type fixes

• CodexAuthMethod.ts: v1 AuthMethod is now a union with terminal methods, whose env has a different shape on v2. Every Codex method is agent-handled, so the constants and getCodexAuthMethods are typed as AuthMethodAgent. No wire change.
• AcpV2Prompt.ts, CodexAcpServer.ts: the v2 idle state is now a union per stop reason, which TypeScript does not match against a union-typed stopReason. toV2IdleState carries the one cast, and the unowned-turn path reuses it. No wire change.
• ACPSessionConnection.ts: startReplayMessage builds one literal per message kind for the same reason. No wire change.
• AcpV2SessionUpdate.ts:
  • command input is tagged without carrying the untagged v1 input type. input: null and an absent input are preserved; only the key order of the rendered object changes, hence the one snapshot update.
  • v1 SessionUpdate gained subagent_update, session_message and session_message_chunk. The adapter does not emit them, so toV2SessionUpdate rejects them like async_task_progress.
  • the comment on _subagent_update said subagent_update was not in the v2 unstable schema yet; it is as of this SDK, so the comment now says the payload does not have its fields yet.

Behavior change to review

SDK 1.5 stripped clientCapabilities.subagents while parsing initialize (noted in #592). SDK 1.8 keeps it, so clientSupportsSubagents now returns true for any v1 client that advertises the capability, not only AIR clients with nativeSubagentSessions. Such a client then receives subagent_spawned / subagent_state_update, while the current RFD defines the capability as understanding subagent_update.

This PR does not change that code. If the earlier-draft updates should stay AIR-only until the v1 path moves to subagent_update, the top-level check in clientSupportsSubagents needs removing in the same release; happy to add that here.

Verification

• npm run typecheck
• npm run build
• npm test (1714 passed, 36 skipped)

🤖 Generated with Claude Code