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/message-agent-executor.ts

@@ -240,7 +240,7 @@ export async function runAgentForGroup(
       phase: 'final',
     });
     completePairedExecutionContext({
-      executionId: pairedExecutionContext.execution.id,
+      taskId: pairedExecutionContext.task.id,
       status: pairedExecutionStatus,
       summary: pairedExecutionSummary,
     });
@@ -940,7 +940,7 @@ export async function runAgentForGroup(
   } finally {
     if (pairedExecutionContext) {
       completePairedExecutionContext({
-        executionId: pairedExecutionContext.execution.id,
+        taskId: pairedExecutionContext.task.id,
         status: pairedExecutionStatus,
         summary: pairedExecutionSummary,
         reviewerVerdict: pairedReviewerVerdict,