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

Start with ROADMAP.md and the design context in docs/design/COMPUTE.md and docs/design/ORCHESTRATOR.md, then locate the orchestration and ComputeStrategy entry points. Review the closed ECS heartbeat issue #45 and related issues #164 and #491. Done means backend-specific regression and matrix tests cover deterministic liveness decisions without cross-runtime false failures, with precedence documented in docs/design/ORCHESTRATOR.md.

Written by the indexing model from the issue text.