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/commit/SKILL.md

@@ -0,0 +1,34 @@
+---
+name: commit
+description: Create a conventional commit for PX4 changes
+disable-model-invocation: true
+argument-hint: "[optional: description of changes]"
+allowed-tools: Bash, Read, Glob, Grep
+---
+
+# PX4 Conventional Commit
+
+Create a git commit: `type(scope): description`
+
+**NEVER add Co-Authored-By lines. No Claude attribution in commits.**
+
+## Steps
+
+1. Check branch (`git branch --show-current`). 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. Run `git status` and `git diff --staged`. If nothing staged, ask what to stage.
+3. Pick **type**: `feat`, `fix`, `docs`, `style`, `refactor`, `perf`, `test`, `build`, `ci`, `chore`, `revert`
+4. Pick **scope** from changed file paths (e.g. `ekf2`, `mavlink`, `sensors`).
+5. Write concise imperative description, lowercase, no period, under 72 chars.
+6. Breaking changes: `type(scope)!: description`
+7. Body (if needed): explain **why**, not what.
+8. Stage and commit. No `Co-Authored-By`.
+
+## Examples
+
+```
+feat(ekf2): add height fusion timeout
+fix(mavlink): correct BATTERY_STATUS_V2 parsing
+refactor(navigator): simplify RTL altitude logic
+```
+
+If the user provided arguments, use them as context: $ARGUMENTS