[TASK] Session manifest lifecycle (.partial -> .json) + startup reconcile

[kirich1409]

Context

Lifecycle: create .json.partial at start, update counts per item, atomic rename to .json on finalize. On startup, orphan .partial files are marked aborted and counts are recomputed from JSONL by sessionId.

Acceptance criteria

• AC-22 passes.
• Crash-injection test produces .partial, then reconcile yields status=aborted and correct counts.

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

• JSONL is authoritative; manifest is derived.

Dependencies

• Blocked by [TASK] RemovalLogger actor + JSONL append path #51

Safety checklist

See .github/ISSUE_TEMPLATE/task.yml — tick applicable invariants in the issue if any destructive path is touched.

Definition of Done

• Acceptance criteria satisfied and verifiable
• Tests added or updated
• No build / lint / test regressions
• Self-review against research-report invariants
• Code review approved