[JEWEL-1260] Add Claude Code skills for YouTrack, PR preparation, and release management

[rock3r]

Summary

Adds three Claude Code agent skills to .claude/skills/, making common Jewel workflows available as invocable agent skills.

Relates to JEWEL-1260

Changes

• managing-youtrack — Interact with the JetBrains YouTrack REST API. Includes security hardening: hardcoded base URL, token passed via stdin (not process args), JSON bodies via temp files (no shell interpolation), anti-prompt-injection rules, and a pre-create review/approval step.
• jewel-pr-preparer — Validates and prepares a Jewel PR end-to-end: checks single-commit requirement, validates [JEWEL-xxx] commit message format, runs all local CI checks (Gradle, detekt, API dumps, Metalava, Bazel), checks for breaking API changes, prompts for screenshots on visual changes, drafts release notes, and optionally creates the PR via gh.
• jewel-release-helper — Interactive step-by-step checklist for the full Jewel release process: version bump, API version code generation, Metalava validation and signature generation, release notes extraction and drafting, commit, cherry-pick to release branches, branch comparison, Maven artifact validation, and tagging.

[DanielSouzaBertoldi]

PR preparer is really awesome, as are the others :)

Thought that maybe we could add a branching path in the release-helper for times when you just want to test a new version locally but such scenarios are pretty rare and they are covered in the releasing-guide.md anyways

[rock3r]

All review comments are addressed — thanks everyone for the feedback!

One note: I am still not done cleaning up these skills overall. The two remaining skills (managing-youtrack and jewel-release-helper) still need a proper pass before this is really ready to merge — I just have not gotten to that yet. Will update when that is done.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Squash command amends master's commit instead of creating new one

High Severity

The squash instructions use git reset --soft master followed by git commit --amend --no-edit. After the reset, HEAD points to master's tip commit, so --amend rewrites that commit rather than creating a new commit on top of it. This produces a commit parented off master's parent, with master's commit message — not the feature branch's first commit message as the surrounding text claims. The correct approach is git commit without --amend, which creates a new single commit on top of master containing only the feature branch changes.