chore(ai): add git rebase skill

.agents/skills/git-rebase/SKILL.md

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+---
+name: git-rebase
+description: Perform quality-assured git rebase with pre/post validation
+metadata:
+  workflow: git
+  audience: developers
+---
+
+## What I do
+
+I perform git rebases while ensuring code quality is maintained throughout the process:
+
+1. **Pre-rebase validation**: Build solution and run all tests using `dotnet nuke test` to establish baseline quality
+2. **Capture baseline warnings**: Record any existing warnings to distinguish them from rebase-introduced issues
+3. **Execute rebase**: Run the rebase command (typically `git pull --rebase github main`, but can target other branches)
+4. **Resolve conflicts**: Help resolve merge conflicts that arise during rebase
+5. **Post-rebase validation**: Re-run `dotnet nuke test` to verify quality is maintained
+6. **Fix new issues**: Address any NEW warnings or test failures introduced by the rebase
+7. **Preserve existing issues**: Leave pre-existing warnings unchanged (they're acceptable)
+
+## When to use me
+
+Use this skill when:
+- Rebasing feature branches onto main or other branches
+- You need confidence that the rebase doesn't break tests or introduce new problems
+- Working in .NET projects that use NUKE build system
+- You want to distinguish between old warnings (OK) and new issues (must fix)
+
+## Command patterns
+
+The most common rebase command is:
+```bash
+git pull --rebase github main
+```
+
+But it can be adapted for other branches:
+```bash
+git pull --rebase origin develop
+git rebase main
+git rebase -i HEAD~5
+```
+
+## Quality gates
+
+**Pre-rebase checks:**
+- Solution must build successfully
+- All tests must pass
+- Document any existing warnings (these are OK to keep)
+
+**Post-rebase requirements:**
+- Solution must still build successfully
+- All tests must still pass
+- No NEW warnings beyond the baseline
+- Any new warnings or failures must be fixed
+
+## Workflow
+
+1. Run `dotnet nuke test` and capture output
+2. Note any warnings (count and types)
+3. Confirm working tree is clean with `git status`
+4. Execute the rebase command
+5. Resolve conflicts as they appear using Edit tool
+6. Continue rebase with `GIT_EDITOR=true git rebase --continue` (non-interactive)
+7. Run `dotnet nuke test` again
+8. Compare warnings: old ones are OK, new ones need fixing
+9. Fix any new issues introduced by the rebase
+10. Verify final build succeeds with same or fewer warnings than baseline