docs: add AGENT.md workflow and local skills

- AGENT.md: role-based issue-driven workflow (PO → Dev → Reviewer)
- CLAUDE.md: points to AGENT.md
- .codex/skills/: validate-issue, do-task, review-pr with phase gates

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.codex/skills/validate-issue/SKILL.md

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+---
+name: validate-issue
+description: Phase 1 skill. Use when a Gitea issue is created or updated and needs requirement validation. Analyze, clarify, and lock scope as Product Owner. Do NOT implement.
+---
+
+# Validate Issue
+
+**Phase:** 1 — Requirement Refinement
+**Role:** Product Owner
+
+## When to invoke
+
+- User says: "check issue #X", "validate issue #X", "/validate-issue #X"
+- A new issue is created or an existing issue is updated with new information.
+
+## Prerequisite
+
+- Issue exists on Gitea.
+
+## Workflow
+
+1. **Load issue**
+   - Read issue: `mcp__gitea__.issue_read` (`method: "get"`).
+   - Read comments: `mcp__gitea__.issue_read` (`method: "get_comments"`).
+
+2. **Analyze**
+   - Is the problem statement clear?
+   - Is the scope well-defined or too broad?
+   - Is it technically feasible?
+   - If needed, read relevant code: `mcp__gitea__.get_file_contents`.
+
+3. **Decide**
+   - **Unclear or incomplete:**
+     - Post clarifying questions: `mcp__gitea__.issue_write` (`method: "add_comment"`).
+     - Stop. Wait for user to respond, then re-run this skill.
+   - **Clear and feasible:**
+     - Post Final Requirements (see format below).
+     - This locks the scope and signals Phase 2 can begin.
+
+## Final Requirements format
+
+```
+## Final Requirements
+
+**Summary:** <one-line summary>
+
+**Scope:**
+- <requirement 1>
+- <requirement 2>
+
+**Out of scope:**
+- <explicitly excluded items>
+
+**Acceptance criteria:**
+- <verifiable check 1>
+- <verifiable check 2>
+```
+
+## Rules
+- Use Gitea MCP tools for all operations.
+- Do NOT write code, create branches, or create PRs.
+- Do NOT proceed to Phase 2 — that is a separate skill.
+
+## Next skill
+After Final Requirements are posted, the user invokes `/do-task <issue-id>` on the Developer agent.