Add periodic checkpoint coordinator state machine

Adds the Idle -> Due -> InFlight -> Idle scheduling loop (with
Finalizing -> Stopped reachable from any state) that will drive
periodic workspace-handoff checkpoints, per REMOTE-2111 Phase 3.

- checkpoint_coordinator.rs: CheckpointCoordinatorHandle spawns the
  loop and exposes finalize() for graceful shutdown. The timer only
  ever moves Idle to Due; Due polls a safe-boundary predicate
  (conversation not mid-turn) before entering InFlight, which runs
  exactly one attempt via snapshot::run_checkpoint_from_declarations_file
  on a background task so a racing finalize() can bound how long it
  waits without ever stranding the attempt. finalize() starts at most
  one more attempt (skipped below the gather/upload timeout floor) or
  awaits an already-in-flight one -- never both -- bounded end to end
  by its budget.
- terminal/view.rs: add TerminalView::ai_action_model(), which
  is_safe_boundary uses to check for unfinished actions on the
  conversation.

This module has no production caller yet -- AgentDriver wires
CheckpointCoordinatorHandle::new into its spawn/finalize lifecycle in a
follow-up, stacked PR -- so it is marked allow(dead_code) at the module
level with a comment explaining why; removable once that PR merges on
top of this one.

Co-Authored-By: Oz <oz-agent@warp.dev>

Author: joeywangzr

app/src/ai/agent_sdk/driver.rs

@@ -99,6 +99,7 @@ use crate::terminal::view::ConversationRestorationInNewPaneType;
 pub(crate) mod attachments;
 #[cfg(feature = "local_fs")]
 pub(crate) mod cache_setup;
+mod checkpoint_coordinator;
 pub(crate) mod cloud_provider;
 pub(crate) mod environment;
 mod error_classification;