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

ROADMAP.md と docs/design/ORCHESTRATOR.md から始め、次に issue で言及されている現在の durable orchestration handler を追跡します。snapshot と action の境界を定義し、decide(snapshot) を網羅的な unit test でカバーして、実行が updated_at の条件付き書き込みを使用し、incremental compatibility を維持していることを検証します。

索引モデルが issue の本文から書いたものです。