anchor rm: remove an anchor worktree and dissolve its anchor

[ed-irl]

Adds gs anchor rm <path>, the counterpart to anchor create/track, for tearing down an anchor worktree once its work is done.

The command resolves the path to an anchor (matching the registry's recorded worktree path, then falling back to the branch checked out there), then:

• Refuses to remove the current worktree up front, since Git won't remove the working tree it's run from.
• Removes the worktree first — the only step that can fail for reasons outside our control (a dirty tree without --force) — so a failure leaves all state untouched and the command is safe to retry.
• Retargets the anchor's direct children onto its base (the pinned branch for an internal anchor, else the canonical trunk) in retarget-only mode, leaving them needing a restack rather than risking a conflict.
• Deletes the anchor's pointer branch and unregisters the anchor.

--force removes a worktree with uncommitted changes.

Includes a test script covering self-removal refusal, the dirty-tree no-op-on-failure guarantee, successful removal with child retargeting, and the non-anchor-path error, plus help golden files.

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • feat: add worktree management commands and per-worktree trunks #1187
    • anchor: Make restack and log anchor-aware #1302
      • anchor rm: remove an anchor worktree and dissolve its anchor #1303 ◀
        • repo: Add exclusive mode (park/restore/exclusive) #1304
          • docs: Add parallel worktrees guide #1305

_{Change managed by git-spice.}