refactor(guardrails): guardrail.ts 1850行 → モジュール分割 (800行制限超過)

[terisuke]

Problem

guardrail.ts は現在 1850+ 行で、プロジェクトの 800 行制限を大幅に超過。
PR #145 で +290 行、PR #146 で +20 行追加し悪化。

Proposed Split

Module	Responsibility	Est. Lines
guardrail-core.ts	State management, helpers, secExempt	~200
guardrail-access.ts	File access control, config protection	~200
guardrail-workflow.ts	Orchestration engine, phase transitions	~250
guardrail-review.ts	Auto-review pipeline, parseFindings	~200
guardrail-git.ts	Branch hygiene, merge gate, CI check	~200
guardrail.ts	Entry point, hook registration	~300

Related

• Code-reviewer MEDIUM-3 finding on PR feat(guardrails): autonomous pipeline — orchestration, auto-review, ship agent #145