feat: add codex-imagegen backend for non-Codex runtimes

Implements the preferred_image_backend: codex-imagegen config key
already referenced in several SKILL.md files but with no corresponding
backend. Bridges non-Codex runtimes (Claude Code, etc.) to Codex CLI's
built-in image_gen tool via 'codex exec'. Uses the user's Codex
subscription — no OPENAI_API_KEY required.

Reliability:
- Retry with exponential backoff (default 2 retries, configurable)
- 9 classified error kinds (retryable vs non-retryable)
- Node child_process timeout with SIGTERM→SIGKILL ladder

Correctness:
- Verifies image_gen was actually invoked (not bypassed) by checking
  $CODEX_HOME/generated_images/{thread_id}/ for a PNG
- PNG magic byte validation
- Output file size sanity check

Observability:
- Structured JSONL logging with --log-file
- Verbose stderr mode with --verbose
- Returns thread_id, tool_calls, token usage in result JSON
- Raw codex event stream preserved per attempt for debugging

Efficiency:
- Idempotency cache via --cache-dir (sha256 of prompt+aspect+refs)
- Cache hit returns in <0.3s vs 50-90s cold

Features:
- --ref reference images (repeatable, passed through to codex exec --image)
- --retries, --retry-delay, --timeout all configurable
- File lock serializes concurrent invocations

Testing:
- 16 Bun unit tests across parser, cache, validator
- Path-filtered CI workflow at .github/workflows/codex-imagegen-tests.yml
- Composes with existing test.yml without conflicts

Architecture:
- scripts/codex-imagegen.sh: thin bash entrypoint (bun → npx -y bun fallback)
- scripts/codex-imagegen/: TypeScript module
  - main.ts (orchestrator), types.ts (CliOptions/GenerateResult/GenError),
    spawn.ts (child_process), parser.ts (JSONL events), validator.ts
    (image_gen check + PNG magic), cache.ts (sha256 cache + FileLock),
    logger.ts (JSONL structured logger)

Trade-offs documented in docs/codex-imagegen-backend.md:
- 5-10x slower than direct OpenAI API (or near-free on cache hit)
- ToS gray area: image_gen is documented for interactive use; non-
  interactive invocation via codex exec is not explicitly addressed.
  Suitable for personal low-volume use; users are responsible for
  ensuring their usage complies with applicable terms of service.

scripts/codex-imagegen/logger.ts

@@ -0,0 +1,39 @@
+import { appendFile, mkdir } from "node:fs/promises";
+import path from "node:path";
+
+export interface LogEntry {
+  ts: string;
+  level: "info" | "warn" | "error";
+  event: string;
+  [k: string]: unknown;
+}
+
+export class JsonLogger {
+  constructor(private logFile: string | null, public verbose: boolean) {}
+
+  async log(level: LogEntry["level"], event: string, extra: Record<string, unknown> = {}): Promise<void> {
+    const entry: LogEntry = { ts: new Date().toISOString(), level, event, ...extra };
+    const line = JSON.stringify(entry);
+    if (this.verbose) process.stderr.write(`[${level}] ${event} ${jsonExtras(extra)}\n`);
+    if (this.logFile) {
+      await mkdir(path.dirname(this.logFile), { recursive: true });
+      await appendFile(this.logFile, line + "\n", "utf-8");
+    }
+  }
+
+  info(event: string, extra?: Record<string, unknown>) {
+    return this.log("info", event, extra);
+  }
+  warn(event: string, extra?: Record<string, unknown>) {
+    return this.log("warn", event, extra);
+  }
+  error(event: string, extra?: Record<string, unknown>) {
+    return this.log("error", event, extra);
+  }
+}
+
+function jsonExtras(extra: Record<string, unknown>): string {
+  const entries = Object.entries(extra);
+  if (entries.length === 0) return "";
+  return entries.map(([k, v]) => `${k}=${typeof v === "string" ? v : JSON.stringify(v)}`).join(" ");
+}