ci(claude): add Claude Code skills for commit, pr, and rebase

Add reusable skill definitions for common contributor workflows:
- commit: creates conventional commits with proper type(scope) format
- pr: creates PRs with conventional commit titles
- rebase-onto-main: handles rebasing onto main when parent branches
  were squash-merged

.claude/skills/rebase-onto-main/SKILL.md

@@ -0,0 +1,73 @@
+---
+name: rebase-onto-main
+description: Rebase a branch onto main, handling squash-merged parent branches cleanly
+argument-hint: "[optional: branch name, defaults to current branch]"
+allowed-tools: Bash, Read, Glob, Grep, Agent
+---
+
+# Rebase Branch onto Main
+
+Rebase the current (or specified) branch onto `main`, correctly handling the case where the branch was built on top of another branch that has since been squash-merged into `main`.
+
+## Background
+
+When a parent branch is squash-merged, its individual commits become a single new commit on `main` with a different hash. A normal `git rebase main` will try to replay the parent's original commits, causing messy conflicts. The fix is to **cherry-pick only the commits unique to this branch** onto a fresh branch from `main`.
+
+## Steps
+
+1. **Identify the branch.** Use `$ARGUMENTS` if provided, otherwise use the current branch.
+
+2. **Fetch and update main:**
+   ```
+   git fetch origin main:main
+   ```
+
+3. **Find the merge base** between the branch and `main`:
+   ```
+   git merge-base <branch> main
+   ```
+
+4. **List all commits** on the branch since the merge base:
+   ```
+   git log --oneline <merge-base>..<branch>
+   ```
+
+5. **Identify which commits are unique to this branch** vs. inherited from a parent branch. Look for:
+   - Squash-merged commits on `main` that correspond to a group of commits at the bottom of the branch's history (check PR titles, commit message keywords).
+   - The boundary commit: the first commit that belongs to *this* branch's work, not the parent's.
+   - If ALL commits are unique (no parent branch), just do a normal `git rebase main` and skip the rest.
+
+6. **Create a fresh branch from `main`:**
+   ```
+   git checkout -b <branch>-rebase main
+   ```
+
+7. **Cherry-pick only the unique commits** (oldest first):
+   ```
+   git cherry-pick <first-unique-commit>^..<branch>
+   ```
+   The `A^..B` range means "from the parent of A through B inclusive."
+
+8. **Handle conflicts** if any arise during cherry-pick. Resolve and `git cherry-pick --continue`.
+
+9. **Replace the old branch:**
+   ```
+   git branch -m <branch> <branch>-old
+   git branch -m <branch>-rebase <branch>
+   ```
+
+10. **Verify** the result:
+    ```
+    git log --oneline main..<branch>
+    ```
+    Confirm only the expected commits are present.
+
+11. **Ask the user** before force-pushing. When approved:
+    ```
+    git push origin <branch> --force-with-lease
+    ```
+
+12. **Clean up** the old branch:
+    ```
+    git branch -D <branch>-old
+    ```