feat(plugin): add worktree skill

[christso]

Summary

• add a new engineering plugin to the bundled marketplace
• add a model-invoked worktree skill with a packaged Bash helper
• default worktrees to <repo>__worktrees/<sanitized-branch>
• support WORKTREE_ROOT, WORKTREE_REMOTE, and -C <repo>
• add and track existing branches, create new branches, and safely remove worktrees without deleting branches

Verification

bash -n plugins/engineering/skills/worktree/scripts/worktree.sh
jq empty plugins/engineering/.claude-plugin/plugin.json .claude-plugin/marketplace.json
bun install --frozen-lockfile

Manual E2E:

1. Initialized a temporary Git repository and AllAgents workspace.
2. Installed plugins/engineering as a local plugin with only the worktree skill enabled.
3. Confirmed the synced script remained executable and ran its help command from the installed skill.
4. Used the packaged script with -C to add and remove feat/example; verified removal preserved the branch.
5. Repeated add/remove against this repository with a custom WORKTREE_ROOT; cleaned the temporary worktree and branch afterward.

No application runtime code changed.

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