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

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

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

Commencez par ROADMAP.md et le contexte de conception dans docs/design/COMPUTE.md et docs/design/ORCHESTRATOR.md, puis localisez les points d’entrée de l’orchestration et de ComputeStrategy. Examinez l’issue fermée concernant le heartbeat ECS, #45, ainsi que les issues associées #164 et #491. Le travail est considéré comme terminé lorsque des tests de régression spécifiques au backend et des tests en matrice couvrent des décisions de liveness déterministes sans faux échecs entre les runtimes, avec la précédence documentée dans docs/design/ORCHESTRATOR.md.