refactor: replace toolset-oriented skills with 27 workflow-oriented skills

Replace 16 toolset-oriented skills (repos, issues, pull-requests, etc.)
with 27 workflow-oriented skills that map to real user intents:

PR workflows: create-pr, review-pr, self-review-pr, address-pr-feedback, merge-pr
Issue workflows: triage-issues, create-issue, manage-sub-issues
CI/CD: debug-ci, trigger-workflow
Security: security-audit, fix-dependabot, research-vulnerability
Code exploration: explore-repo, search-code, trace-history
Projects: manage-project
Notifications: handle-notifications
Releases: prepare-release
Repository management: manage-repo, manage-labels
Collaboration: contribute-oss, browse-discussions, delegate-to-copilot
Discovery: discover-github, share-snippet, get-context

Each skill focuses on a specific workflow with non-obvious guidance,
anti-patterns, and identity-aware instructions (e.g., reviewing someone
else's PR vs self-reviewing your own). Tools overlap across skills
based on workflow needs rather than API domain.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SamMorrowDrums