Add release-notes skill for generating release notes

[ChrisJBurns]

Summary

Manually generating release notes is time-consuming and error-prone — breaking changes get missed, migration guides are inconsistent, and dependency PRs get mixed with features. This adds a /release-notes skill that automates the entire process by analyzing every PR between two tags, dispatching domain-expert agents (kubernetes-expert, oauth-expert, mcp-protocol-expert) to assess breaking changes, and composing publication-ready notes from a consistent template.

Type of change

• New feature (non-breaking change which adds functionality)

Test plan

• Tested by running /release-notes https://github.com/stacklok/toolhive/releases/tag/v0.20.0 end-to-end, which produced complete release notes with expert-validated breaking change assessments

Does this introduce a user-facing change?

Adds /release-notes <tag-or-url> slash command for generating structured release notes with automated breaking-change detection.

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.10%. Comparing base (bfac267) to head (085be48).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4852      +/-   ##
==========================================
+ Coverage   69.07%   69.10%   +0.02%     
==========================================
  Files         531      531              
  Lines       55170    55170              
==========================================
+ Hits        38110    38126      +16     
+ Misses      14132    14115      -17     
- Partials     2928     2929       +1     

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