P4: checkpoint battery — durable-resume seam + run.Checkpointer handle

Plugs into run.Ports.Checkpointer (the executor's call site is a P2 follow-up;
this provides the seam + impls ahead of it):
- checkpoint.go: CheckpointStore seam + RunCheckpoint{Meta, Messages, Iteration,
  ActivePhase} + RunCheckpointMeta (mirrors mort's agentexec types).
- handle.go: New(store, meta, throttle, now) -> run.Checkpointer. Save writes a
  throttled snapshot; Complete/Fail delete it (a cleanly finished or terminally
  failed run is NOT a recovery candidate; a shutdown-interrupted run never calls
  them, so its checkpoint survives ListInterrupted at boot). nil store -> no-op.
- memory.go: NewMemory() default (with the honest caveat that in-memory does
  not survive the restart it exists to recover from — a durable store is mort's).

Tests: save+complete clears the recovery candidate; throttle skips in-window
saves; nil-store is a clean no-op. Core imports ZERO from checkpoint.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -71,7 +71,8 @@ BATTERIES (opt-in siblings, each nil-safe + a default):
              default (skillaudit Storage iface; GORM stays in mort)
   critic/    two-tier timeout state machine + Escalator  [P4]
   schedule/  cron runner cores                           [P4]
-  checkpoint/ durable resume seam                        [P4]
+  checkpoint/ CheckpointStore + run.Checkpointer handle  [P4 ✓]
+             (throttled Save/Complete/Fail) + Memory (exec wiring=P2 follow-up)
   budget/    DBBudget rolling-7d + NoOp (run.Budget);    [P4 ✓]
              BudgetStorage iface + Memory default