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

src/config.ts

@@ -87,6 +87,14 @@ export const RECOVERY_CONCURRENT_AGENTS = parseInt(
   getEnv('RECOVERY_CONCURRENT_AGENTS') || '3',
   10,
 );
+
+// ── Container isolation ───────────────────────────────────────────
+export const REVIEWER_CONTAINER_IMAGE =
+  getEnv('REVIEWER_CONTAINER_IMAGE') || 'ejclaw-reviewer:latest';
+export const CREDENTIAL_PROXY_PORT = parseInt(
+  getEnv('CREDENTIAL_PROXY_PORT') || '3001',
+  10,
+);
 export const RECOVERY_STAGGER_MS = parseInt(
   getEnv('RECOVERY_STAGGER_MS') || '2000',
   10,