feat: add wt-update, wt-destroy tasks and fetch-before-add

[davidpoblador]

Summary

• wt-add now fetches all remotes before branching, so new worktrees always start from fresh remote state. Skips gracefully when no remotes are configured.
• wt-update (new task): fetches all remotes and fast-forwards the main worktree via --ff-only.
• wt-destroy (new task): removes worktree, deletes local branch, and deletes the remote branch in one shot. Refuses to touch main.
• Updated README and AGENTS.md with the new tasks, linked the followup post git-wt: Worktrees Simplified, and expanded credits for Ahmed el Gabri.

Inspired by git-wt.

Test plan

• Scaffold a new project with a remote URL, verify wt-add fetches before creating the worktree
• Scaffold a new project without a remote URL, verify wt-add skips fetch without errors
• Run wt-update with a remote configured, verify main fast-forwards
• Run wt-update without remotes, verify clean error message
• Run wt-destroy on a branch with a remote counterpart, verify all three (worktree, local, remote) are removed
• Run wt-destroy on a local-only branch, verify worktree and local branch removed with appropriate message
• Run wt-destroy main, verify it refuses

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