Merge git and github skills into single git skill

[vredchenko]

Description

Merge the separate git and github skills into a single unified git skill, and enhance it with SmartEM-specific context that's already available in core/.

Current State

• claude-code/shared/skills/git/SKILL.md - git operations (commits, branches, rebasing, etc.)
• claude-code/shared/skills/github/SKILL.md - GitHub operations (PRs, issues, CI/CD, gh CLI)

Proposed Changes

1. Merge skills - Combine both into git/SKILL.md (git operations first, then GitHub/gh CLI)

2. Add SmartEM project context section with:

   • Reference to core/github-labels.json for available labels
   • Project board ID 51 (defined in core/repos.json under links.projectBoard)
   • Quick lookup table of commonly used labels

3. Delete claude-code/shared/skills/github/ directory

4. Update workspace symlinks if needed

Context from core/

Labels (core/github-labels.json):

• Types of work: documentation, testing, bugfixing, development, refactoring, research, devops, security, admin, enhancement
• System components: smartem-backend, smartem-backend:db, smartem-backend:api, smartem-agent, smartem-frontend, smartem-aria-connector, smartem-devtools, smartem-devtools:webui, smartem-devtools:claude, smartem-devtools:e2e-test

Project Board (core/repos.json):

• ID: 51
• URL: https://github.com/orgs/DiamondLightSource/projects/51/views/1
• Owner: DiamondLightSource

Open Questions

1. Should labels be embedded in SKILL.md or just reference the JSON file?
2. Any symlink updates needed in workspace .claude/skills/?