refactor: simplify paired review system and add reviewer container isolation

Phase 1 — Strip over-engineering (-3,665 lines):
- DB tables: 7 → 3 (remove executions, approvals, artifacts, events)
- Task statuses: 11 → 5 (active, review_ready, in_review, merge_ready, completed)
- Remove plan governance, event sourcing, gate/verdict, freshness guards
- paired-execution-context: 980 → 299 lines
- Session commands: remove /risk, /plan, /approve-plan, /request-plan-changes

Phase 2 — Container isolation for reviewers:
- Add container-runtime.ts (Docker abstraction)
- Add credential-proxy.ts (API key injection without container exposure)
- Add container-runner.ts (reviewer-specific read-only mount + tmpfs)
- Add container/Dockerfile + agent-runner (Chromium, Claude Code, Codex)
- agent-runner.ts: auto-route reviewer execution to container mode

container/agent-runner/package.json

@@ -0,0 +1,17 @@
+{
+  "name": "ejclaw-reviewer-runner",
+  "version": "1.0.0",
+  "type": "module",
+  "description": "Reviewer agent runner for EJClaw containers",
+  "main": "dist/index.js",
+  "scripts": {
+    "build": "tsc",
+    "dev": "tsc --watch"
+  },
+  "dependencies": {
+    "@anthropic-ai/claude-code": "^1.0.0"
+  },
+  "devDependencies": {
+    "typescript": "^5.7.0"
+  }
+}