feat(durable): Phase 2 swarm-tree checkpointing

[cdayAI]

Phase 2 of docs/specs/durable-execution.md, building on the episode-scoped keying from #497. Closes the gap Phase 1 left for swarm goals: a crash during spawn_swarm (children in-flight, no tool_result recorded) used to re-run the entire swarm on resume. Now each child resumes its own loop.

What's here

• Agent gains an optional checkpoint_id override + _current_step. The depth-0 gate in run() is lifted so any agent with an explicit checkpoint_id (a swarm child) also checkpoints. Children resume their own messages+step but do not restore the shared budget — only the depth-0 owner does (a child restoring would clobber the swarm's shared counter).
• spawn_swarm keys each child by parent.checkpoint_id + spawn-step + index + brief-hash (stable across resume). After gather, children that returned are cleared (their finals are already in the parent's history); a child that raised keeps its checkpoint for resume.
• checkpoint.clear_agent(goal_id, agent_id, episode_id) for per-child cleanup.

Soundness (the careful part)

Resume is continuation, not replay — a resumed parent re-decides and may spawn a different set of children. The keying handles both cases correctly:

• Identical re-spawn → each child's key matches its prior checkpoint → resumes mid-loop. ✓
• Divergent re-spawn → new keys → fresh children; the orphaned checkpoints are pruned/cleared. ✓

Deferred (documented): skipping a fully-completed child entirely (memoizing its final across a parent re-decision) needs spawn-intent replay — not done here. The parent still re-runs the swarm step, just without re-running finished children's full loops. This is the correct, sound slice; the memoization is a further optimization.

Verification

• Off by default, fail-open. Full suite 2481 passed, 0 regressions; 163 spawn/swarm tests green (the hot path this touches).
• New tests: clear_agent scoping; explicit child checkpoint_id; and an end-to-end spawn_swarm where one child finishes and one crashes — the crashed child keeps its checkpoint, the finished sibling's is cleared.

https://claude.ai/code/session_01V4m74QKcM4ERqAu3rbkr6B

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