ci: validate plugins in CI, and land the Codex plugin

[minhthanhdang]

🚥 Resolves RM-18445
🚥 Resolves RM-18419

🧰 Changes

Adds a GitHub Actions workflow that runs Claude Code's built-in plugin validation on every PR and push to main, covering both the plugin folder and the root marketplace. It runs in strict mode, so warnings fail the check too. A broken manifest, marketplace entry, or skill frontmatter now blocks the merge instead of surfacing at install time.

One gap worth knowing: the validator does not check that a marketplace entry's source path exists, so a marketplace pointing at a missing folder still passes.

Also carries the Codex plugin and the root marketplace, so codex plugin marketplace add readmeio/agent-plugins then codex plugin add readme@readme installs it. That work was reviewed and approved in #3, which landed on the wrong base and never reached main — this re-targets it rather than reopening.

🧬 QA & Testing

• Summary Report: n/a. Validation fails on a skill with no frontmatter and passes once it is restored. Codex install verified in an isolated CODEX_HOME on 0.154.0: plugin shows installed, enabled at 1.0.0 with its skills, MCP config, and icons.

(I merged #3 into wrong branch so this PR carries #3 changes as well)

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; the incremental change fully fixes the remaining invalid skill handoff and introduces no new actionable issue.

_{Reviews (8) · Last reviewed commit: "docs(onboarding): drop the pointer to a ..."}

Dismissed because a newer commit was pushed; Greptile will re-review the current head.

Dismissed because a newer commit was pushed; Greptile will re-review the current head.

Dismissed because a newer commit was pushed; Greptile will re-review the current head.