feat(orchestration): unified liveness decision model across compute backends

**Context:** ROADMAP.md → Unified liveness decision model
**Related:** #164, #45 (closed ECS heartbeat bug)

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## Component

API or orchestration

## Describe the feature

Normalize **task health evaluation** across compute backends (AgentCore, ECS) through a single typed decision path: heartbeat support, compute session status, DynamoDB state—with deterministic precedence and regression tests preventing cross-runtime false failures.

## Use case

#45 showed ECS tasks marked FAILED after success due to heartbeat mismatch. Adding backends multiplies liveness edge cases without a unified model.

## Proposed solution

1. `LivenessCapabilities` per `ComputeStrategy` (supports_heartbeat, session_status_api, etc.).
2. Pure function: `(snapshot, capabilities) → LivenessDecision`.
3. Orchestrator uses single path; unit tests per backend + matrix tests.
4. Document precedence in `docs/design/ORCHESTRATOR.md`.

## Other information

- Prerequisite for reliable **alternative compute** (#164).
- Design context: `docs/design/COMPUTE.md`, `docs/design/ORCHESTRATOR.md`.

- [ ] This might be a breaking change

Inizia da ROADMAP.md e dal contesto di progettazione in docs/design/COMPUTE.md e docs/design/ORCHESTRATOR.md, quindi individua gli entry point dell’orchestrazione e di ComputeStrategy. Esamina l’issue chiusa sull’heartbeat ECS, #45, e le issue correlate #164 e #491. Il lavoro è completato quando i test di regressione specifici per il backend e i test a matrice coprono decisioni deterministiche di liveness senza falsi errori tra runtime, con la precedenza documentata in docs/design/ORCHESTRATOR.md.

Scritto dal modello di indicizzazione a partire dal testo della issue.