test: cover recovery after Pi retry

[c0da-ricon]

Summary

• add a real SessionManager regression for Pi's persisted auto-retry shape
• prove one visible user/assistant pair can validly map to user, retryable assistant error, and successful assistant entries
• verify restart selects the exact committed leaf without interpreting raw Pi message cardinality

Why

PR #9 correctly removes role-count matching from recovery, but needs a regression demonstrating the valid runtime shape that motivates removing that guard.

Validation

• focused recovery test: 6/6 passed
• full equivalent check: strict TypeScript, 59/59 tests, all six Codebase lints, and whitespace checks
• git pre-commit
• git pre-push (expected new-branch and locally unknown GitHub merge-signature warnings only)

Test-only change. No model call, service start, deployment, or merge.