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

[krokoko]

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 (refactor(compute): gate ECS construct on compute_type context instead of comment toggle #164).

• Design context: docs/design/COMPUTE.md, docs/design/ORCHESTRATOR.md.

• This might be a breaking change