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."

[ClintEastman02]

Good call — added the merged-branch detection in fe30fb7. Worktrees whose HEAD is already an ancestor of origin/main now get reported with a MERGED line and skipped (no-op rebase avoided), so the operator knows the worktree is ready to prune.

Test matrix (all four cases verified locally):

• Conflicting commit on a stale base → FAIL + git rebase --abort, non-zero exit
• Branch already merged into origin/main → MERGED, skipped
• Stale-and-clean (unique commits behind main) → rebased
• Stale-and-dirty (uncommitted changes) → SKIP
• main worktree itself → skipped silently

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #218   +/-   ##
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  Coverage        ?   85.98%           
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  Files           ?      167           
  Lines           ?    39535           
  Branches        ?     3795           
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  Hits            ?    33993           
  Misses          ?     5542           
  Partials        ?        0           

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