feat: add working-in-parallel bundled skill (#577)

Co-authored-by: Letta <noreply@letta.com>

src/skills/builtin/working-in-parallel/SKILL.md

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+---
+name: working-in-parallel
+description: Guide for working in parallel with other agents. Use when another agent is already working in the same directory, or when you need to work on multiple features simultaneously. Covers git worktrees as the recommended approach.
+---
+
+# Working in Parallel
+
+Use **git worktrees** to work in parallel when another agent is in the same directory.
+
+Git worktrees let you check out multiple branches into separate directories. Each worktree has its own isolated files while sharing the same Git history and remote connections. Changes in one worktree won't affect others, so parallel agents can't interfere with each other.
+
+Learn more: [Git worktree documentation](https://git-scm.com/docs/git-worktree)
+
+## Quick Start
+
+```bash
+# Create worktree with new branch (from main repo)
+git worktree add -b fix/my-feature ../repo-my-feature main
+
+# Work in the worktree
+cd ../repo-my-feature
+bun install  # or npm install, pip install, etc.
+
+# Make changes, commit, push, PR
+git add <files>
+git commit -m "fix: description"
+git push -u origin fix/my-feature
+gh pr create --title "Fix: description" --body "## Summary..."
+
+# Clean up when done (from main repo)
+git worktree remove ../repo-my-feature
+```
+
+## Key Commands
+
+```bash
+git worktree add -b <branch> <path> main  # Create with new branch
+git worktree add <path> <existing-branch>  # Use existing branch
+git worktree list                          # Show all worktrees
+git worktree remove <path>                 # Remove worktree
+```
+
+## When to Use
+
+- Another agent is working in the current directory
+- Long-running task in one session, quick fix needed in another
+- User wants to continue development while an agent works on a separate feature
+
+## Tips
+
+- Name directories clearly: `../repo-feature-auth`, `../repo-bugfix-123`
+- Install dependencies in new worktrees (`npm install`, `bun install`, `pip install`, etc.)
+- Push changes before removing worktrees
+
+## Alternative: Repo Clones
+
+Some users prefer cloning the repo multiple times (`gh repo clone owner/repo project-01`) for simpler mental model. This uses more disk space but provides complete isolation. If the user expresses confusion about worktrees or explicitly prefers clones, use that approach instead.