feat(worker_boot): classify session completion failures (P2.13)

Add WorkerFailureKind::Provider variant and observe_completion() method
to classify degraded session completions as structured failures.

- Detects finish='unknown' + zero tokens as provider failure
- Detects finish='error' as provider failure
- Normal completions transition to Finished state
- 2 new tests verify classification behavior

This closes the gap where sessions complete but produce no output,
and the failure mode wasn't machine-readable for recovery policy.

ROADMAP P2.13 backlog item added.

ROADMAP.md

@@ -287,6 +287,7 @@ Priority order: P0 = blocks CI/green state, P1 = blocks integration wiring, P2 =
 10. MCP structured degraded-startup reporting
 11. Structured task packet format
 12. Lane board / machine-readable status API
+13. **Session completion failure classification** — sessions that complete with `finish: "unknown"` or zero output (provider errors, context exhaustion, etc.) are not classified by `WorkerFailureKind`; worker boot state machine cannot surface these as structured failures for recovery policy
 
 **P3 — Swarm efficiency**
 13. Swarm branch-lock protocol — detect same-module/same-branch collision before parallel workers drift into duplicate implementation