feat(ci): add Copilot full automation pipeline

- copilot-assign.yml: GraphQL agentAssignment
- ai-review.yml: Claude Opus auto-review
- auto-merge.yml: CI + APPROVE squash merge
- decompose.yml: Issue decomposition + sub-issues

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/copilot-assign.yml

@@ -0,0 +1,78 @@
+name: Auto-assign Copilot Coding Agent
+
+# copilot ラベルが付いた Issue に GitHub Copilot Coding Agent を GraphQL でアサインする。
+# 正しい API: GraphQL replaceActorsForAssignable + agentAssignment フィールド
+# (REST API の assignees や @github-copilot コメントは Coding Agent を起動しない)
+on:
+  issues:
+    types: [labeled, opened]
+
+jobs:
+  assign:
+    name: Assign Copilot Coding Agent
+    runs-on: ubuntu-latest
+    # copilot ラベルが付いている Issue のみ対象
+    if: contains(github.event.issue.labels.*.name, 'copilot')
+    permissions:
+      issues: write
+    steps:
+      - name: Check if Copilot already assigned
+        id: check
+        uses: actions/github-script@v7
+        with:
+          result-encoding: string
+          script: |
+            const timeline = await github.rest.issues.listEventsForTimeline({
+              owner:        context.repo.owner,
+              repo:         context.repo.repo,
+              issue_number: context.issue.number,
+              per_page:     100,
+              mediaType:    { previews: ['mockingbird'] },
+            });
+            // "assigned" イベントで assignee.login === "Copilot" が存在すれば既に割り当て済み
+            const already = timeline.data.some(
+              e => e.event === 'assigned' && e.assignee?.login === 'Copilot'
+            );
+            return already ? 'yes' : 'no';
+
+      - name: Assign Copilot via GraphQL
+        if: steps.check.outputs.result == 'no'
+        uses: actions/github-script@v7
+        with:
+          # GITHUB_TOKEN で動作（PAT 不要）
+          # 修正: agentAssignment.targetRepositoryId が必須。ないと silent fail する。
+          script: |
+            const issueNodeId = context.payload.issue.node_id;
+
+            // リポジトリの Node ID を取得（agentAssignment に必須）
+            const repoData = await github.graphql(`
+              query GetRepoId($owner: String!, $name: String!) {
+                repository(owner: $owner, name: $name) { id }
+              }
+            `, {
+              owner: context.repo.owner,
+              name: context.repo.repo,
+            });
+            const repoId = repoData.repository.id;
+
+            await github.graphql(`
+              mutation AssignCopilot($input: ReplaceActorsForAssignableInput!) {
+                replaceActorsForAssignable(input: $input) {
+                  assignable {
+                    ... on Issue { number title }
+                  }
+                }
+              }
+            `, {
+              input: {
+                assignableId: issueNodeId,
+                actorLogins: [],
+                agentAssignment: {
+                  targetRepositoryId: repoId,   // 必須: これがないと Copilot が起動しない
+                  baseRef: 'master',
+                  customInstructions: 'IMPORTANT: Every PR MUST contain at least one executable code file (.js .ts .py .sh .go .rs etc). Do NOT create PRs with only .md documentation files — that is not acceptable. Implement exactly what the Issue acceptance criteria require. Write tests for new functions. Keep the PR title concise and never include [WIP]. Make small focused commits. If the Issue is unclear, implement the minimal working version that satisfies the stated acceptance criteria.',
+                },
+              },
+            });
+
+            console.log(`✅ Copilot Coding Agent assigned to #${context.issue.number}`);