MM-26: Add bounded provider-overflow recovery

Parent

#771

Checkpoint

MC-8 Context recovery

What to build

Classify prompt-too-long errors and run bounded emergency recovery that removes old tool bodies or selects source-bearing compact input without mutating the transcript.

Acceptance criteria

• Recovery retries are capped.
• Recent unresolved tool results remain protected.
• Emergency changes do not modify RuntimeEvent history.
• Invalid recovery summaries fail explicitly.
• Normal strategy and emergency metrics are separate.

Required tests

• Provider error variants
• retry cap
• recent tool protection
• invalid summary
• impossible recovery

Benchmark

Fault-injected long tool loops; recovery >=95%, normal benchmark overflow zero.

Blocked by

• [MM-16] Add bounded working-state projection and resume contract #795 (MM-16)
• [MM-18] Add bounded archive retrieval, search refs, retention, and tombstones #798 (MM-18)

Rollout and rollback

Disable emergency summary and use deterministic bounded failure handling.