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/pr/SKILL.md

@@ -0,0 +1,22 @@
+---
+name: pr
+description: Create a pull request with conventional commit title and description
+disable-model-invocation: true
+argument-hint: "[optional: target branch or description]"
+allowed-tools: Bash, Read, Glob, Grep
+---
+
+# PX4 Pull Request
+
+**No Claude attribution anywhere (no Co-Authored-By, no "Generated with Claude").**
+
+## Steps
+
+1. Check branch. If on `main`, create a feature branch. Use `<username>/<description>` format where `<username>` comes from `gh api user --jq .login`. If unavailable, just use `<description>`.
+2. Gather context: `git status`, `git log --oneline main..HEAD`, `git diff main...HEAD --stat`, check if remote tracking branch exists.
+3. PR **title**: `type(scope): description` — under 72 chars, describes the overall change across all commits. This becomes the squash-merge commit message.
+4. PR **body**: brief summary + bullet points for key changes. No filler.
+5. Push with `-u` if needed, then `gh pr create`. Default base is `main` unless user says otherwise.
+6. Return the PR URL.
+
+If the user provided arguments, use them as context: $ARGUMENTS