refactor(orchestration): pure decision-function orchestrator with CAS

**Context:** ROADMAP.md → Pure decision function orchestrator refactor
**Related:** autonomous-feedback-loop draft

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## Component

API or orchestration

## Describe the feature

Extract orchestrator decision logic into **pure functions**: `(frozen_snapshot) → typed_action`. Side-effect execution applies actions with **CAS guards** on DynamoDB `updated_at` to prevent stale writes. Prerequisite for autonomous feedback loop.

## Use case

Orchestrator logic is hard to unit test with I/O mocks. Competing durable workers risk race conditions. Complex state machines (PR watcher) need a testable core.

## Proposed solution

1. Define `OrchestratorSnapshot` and `OrchestratorAction` types.
2. Pure `decide(snapshot): Action` with exhaustive unit tests (no AWS mocks).
3. Executor applies action with `updated_at` conditional writes.
4. Incremental refactor from current durable handlers—no big-bang.
5. Document in `docs/design/ORCHESTRATOR.md`.

## Other information

- Explicit prerequisite for **autonomous feedback loop** draft.
- Design context: `docs/design/ORCHESTRATOR.md`.

- [x] This might be a breaking change

Start with ROADMAP.md and docs/design/ORCHESTRATOR.md, then trace the current durable orchestration handlers mentioned in the issue. Define the snapshot and action boundaries, cover decide(snapshot) with exhaustive unit tests, and verify that execution uses updated_at conditional writes while preserving incremental compatibility.

Written by the indexing model from the issue text.