docs: add Rstack release skill

.agents/skills/release-rstack/SKILL.md

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+---
+name: release-rstack
+description: Create a release pull request for the `rstack` npm package at a specific version. Use when asked to prepare, create, or open an rstack package release PR.
+---
+
+# Release Rstack
+
+## Input
+
+- Target version without a leading `v`, for example `1.2.0`.
+
+If the version is missing, ask for it before making changes.
+
+## Steps
+
+1. Check the worktree with `git status --short`. If there are uncommitted changes or untracked files, stop and ask the user how to proceed. Do not stash, discard, or include them.
+
+2. Confirm the current branch is the repository's default branch. Set the release branch to `release/v<version>` and check both local and remote branches. If it already exists, stop and ask the user how to proceed.
+
+3. Create and switch to `release/v<version>` from the clean default-branch HEAD.
+
+4. Update only the `version` field in `packages/rstack/package.json` to `<version>`.
+
+5. Review the diff and confirm it contains exactly the one version-field change above.
+
+6. Create a commit with this exact message: `release: v<version>`.
+
+7. Push the branch to `origin`. Recheck that the branch being pushed is `release/v<version>` and never push the default branch directly.
+
+8. Create a pull request against the default branch. In Codex, use the GitHub connector/plugin; use another available GitHub workflow only when the connector is unavailable. Use `release: v<version>` as the PR title.
+
+Return the pull request URL.