Add a release-scope agent skill

[lesnik512]

Adds skills/release-scope/SKILL.md, an agent skill that runs release-scope through uvx and answers release questions from the report. Install with npx skills add modern-python/release-scope; npx skills discovers skills/<name>/SKILL.md.

• Runs uvx --from 'release-scope>=0.3,<0.4' release-scope: patch releases arrive automatically, and a flag or schema change needs a skill update in the same PR. AGENTS.md now says so.
• Takes --project from git remote get-url origin for the current repository; --group/--project or --jira otherwise.
• Checks settings by listing variable names only, and stops with the missing names instead of asking for a token.
• Writes report, page and cache to ${XDG_CACHE_HOME:-$HOME/.cache}/release-scope, outside the user's repository.
• Documents exit codes 0 to 4 (checked against _errors.py) and which report fields to read, including release.state.
• The README gains an "Agent skill" section.

skills/ is not in the wheel or sdist (checked with uv build).

Dry-run evals (3 prompts, commands only, with and without the skill): 100% vs 85% pass, 28s vs 76s. Baselines missed the version range and one wrote the report into the working directory.