feat(codex-imagegen): cwd-drift immunity (path resolve + skip-git-repo-check)

Two improvements so the wrapper works the same regardless of caller cwd
or whether cwd is a git repo:

1. main.ts: resolve all filesystem path args (--prompt-file, --ref,
   --cache-dir, --log-file) to absolute paths up front. Previously only
   --image was resolved. Agents passing relative paths from arbitrary
   working directories (e.g. when SKILL.md is interpreted from a plugin
   install dir) now produce correct file lookups.

2. spawn.ts: pass --skip-git-repo-check to 'codex exec'. Without it,
   codex refuses to run outside a trusted directory:
   'Not inside a trusted directory and --skip-git-repo-check was not
   specified.' This made the wrapper fail when invoked from /tmp or
   from a non-git project tree.

3. baoyu-cover-image/SKILL.md: explicit path resolution note —
   scripts/codex-imagegen.sh lives at plugin/repo root (not shell
   cwd-relative). From the skill base directory it is at
   '../../scripts/codex-imagegen.sh'. Agents should resolve to an
   absolute path before invoking.

Verified:
- 16 unit tests pass
- e2e from /tmp (non-git, relative-path args) → 45s, 1.6MB PNG, status:ok

Composes with the existing cover-image wiring (commit e3932e4).

scripts/codex-imagegen/spawn.ts

@@ -16,7 +16,16 @@ export async function runCodexExec(input: SpawnInput): Promise<CodexRunResult> {
   const logDir = await mkdtemp(path.join(tmpdir(), "codex-imggen-"));
   const rawLogPath = path.join(logDir, "stream.jsonl");
 
-  const args = ["exec", "--json", "--sandbox", "danger-full-access"];
+  // --skip-git-repo-check: lets the wrapper run from non-git cwds
+  //   (e.g. /tmp, or a skill installed under ~/.claude/plugins/...).
+  //   Without it, codex refuses with "Not inside a trusted directory".
+  const args = [
+    "exec",
+    "--json",
+    "--sandbox",
+    "danger-full-access",
+    "--skip-git-repo-check",
+  ];
   for (const img of input.refImages ?? []) {
     args.push("--image", img);
   }