[M2] Stale memory failure injector (MemoryGuard)

[askmy-stack]

Summary

• Adds AgentMemory with checkpoint/rollback for scripted agents.
• Implements StaleMemoryInjector (shared FailureInjector protocol) that poisons target_plan_id at the trigger step without exceptions.
• Adds stale-memory-001 case YAML with root cause stale_memory.
• Extends ScriptedApiAgent to read the upgrade target from memory.
• Adds memory_live_conflict feature (memory plan vs live task target) as the minimal detector signal.

Test plan

• make ci (32 tests)
• Injector protocol + trigger overwrite unit tests
• End-to-end scripted episode fails validation after stale inject
• Case registry loads stale-memory-001

Fixes #15
Related to #45

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