feat(dx): add worktree:sync mise task to rebase clean worktrees on origin/main

[ClintEastman02]

Summary

Adds a new mise task mise run worktree:sync that fetches origin/main and rebases every clean linked worktree onto it. Implements Option A from #197.

• Iterates git worktree list --porcelain and rebases each linked worktree
• Skips dirty worktrees (uncommitted changes) with a SKIP message
• Skips main/master (rebase isn't the right tool for the primary branch)
• On rebase conflict, runs git rebase --abort and exits non-zero so the operator surfaces the conflict explicitly rather than leaving a half-rebased state

Also updates AGENTS.md to point contributors at the new task in two places (the worktree common-mistake bullet and the Commands you can use list).

Closes #197

Test plan

Tested locally with four worktree states:

• Clean worktree behind origin/main → rebased successfully
• Clean worktree even with origin/main → no-op (rebase reports "up to date")
• Dirty worktree (uncommitted changes) → SKIP message, not touched
• Worktree with conflicting commit → FAIL reported, git rebase --abort runs, working tree restored to pre-rebase HEAD, task exits non-zero
• main worktree → skipped silently
• TOML parses cleanly (mise tasks --all lists //:worktree:sync)
• gitleaks clean on the diff

[scottschreckengaust]

Works as designed. I noticed that the script could be enhanced with a "detect already-merged branches" check (e.g., git branch --merged origin/main would catch branches that already merged), but that's a feature enhancement, not a bug. As-is, it correctly reports "I can't rebase this, you need to look at it."