git worktree: add sync command for git worktrees

[calebdw]

Add jj git worktree sync command that reconciles jj workspace state
with Git worktree state in colocated repositories. It adopts
externally-created Git worktrees as jj workspaces, forgets workspaces
whose Git worktrees have been removed, and repairs paths for moved
worktrees.

Add git.auto-sync-worktrees config (default: false) to opt into
running this synchronization automatically on every jj invocation.
When disabled (the default), users can run jj git worktree sync
manually or use jj git worktree adopt for individual worktrees.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.

[yuja]

I haven't reviewed the implementation yet, but since this isn't a backend-agnostic command, it seems better to move it under the jj git namespace (e.g. jj git workspace import).