chore: add a shared release skill and candidate verifier

[tisonkun]

Summary

• Move the release procedure into a shared .agents/skills/release skill with phase guides, discoverable through AGENTS.md. Remove the standalone RELEASE.md so release managers and agents use the same maintained procedure.
• Add a scoped Codex release_verifier for independent checks of existing candidate artifacts. It follows the shared verification guide and returns evidence; the main agent coordinates publication and the existing license-audit capability handles licensing.
• Document established infrastructure, including the completed crates.io Trusted Publishing setup, in a reference loaded only when needed. Keep skill links inside the skill bundle and resolve cross-capability references by name and explicit repository root.

Validation: cargo x lint, skill validation, local links and table alignment, shell example syntax, and ASF headers. A native Codex run dispatched the configured verifier against two local signed archive fixtures: the matching archive passed, and an archive with a modified README was identified despite valid signatures and checksums. This smoke test covered candidate identity and source verification; it did not build, package, or publish a release.