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/types.ts

@@ -0,0 +1,77 @@
+export interface CliOptions {
+  prompt: string;
+  outputPath: string;
+  aspect: string;
+  refImages: string[];
+  timeoutMs: number;
+  retries: number;
+  retryDelayMs: number;
+  cacheDir: string | null;
+  logFile: string | null;
+  verbose: boolean;
+}
+
+export interface ToolCall {
+  id: string;
+  tool: string;
+  status: string;
+  command?: string;
+}
+
+export interface TokenUsage {
+  input: number;
+  cached_input: number;
+  output: number;
+  reasoning: number;
+}
+
+export interface CodexRunResult {
+  threadId: string | null;
+  toolCalls: ToolCall[];
+  agentMessage: string | null;
+  usage: TokenUsage | null;
+  rawLogPath: string;
+  durationMs: number;
+}
+
+export interface GenerateResult {
+  status: "ok" | "error";
+  path: string;
+  bytes: number;
+  elapsed_seconds: number;
+  thread_id: string | null;
+  attempts: number;
+  cached: boolean;
+  usage: TokenUsage | null;
+  tool_calls: { tool: string; status: string }[];
+  error?: string;
+  error_kind?: ErrorKind;
+}
+
+export type ErrorKind =
+  | "codex_not_installed"
+  | "invalid_args"
+  | "prompt_file_missing"
+  | "spawn_failed"
+  | "timeout"
+  | "no_image_gen_tool_use"
+  | "output_missing"
+  | "invalid_png"
+  | "agent_refused"
+  | "lock_busy";
+
+export const RETRYABLE: ReadonlySet<ErrorKind> = new Set([
+  "spawn_failed",
+  "timeout",
+  "no_image_gen_tool_use",
+  "output_missing",
+  "invalid_png",
+  "agent_refused",
+]);
+
+export class GenError extends Error {
+  constructor(public kind: ErrorKind, message: string, public retryable?: boolean) {
+    super(message);
+    this.retryable = retryable ?? RETRYABLE.has(kind);
+  }
+}