feat(workspace): add worktree utility commands (root, move, lock, unlock, repair)

[reckerp]

What

Adds five worktree utility commands under the gx workspace namespace, implementing section 7 (Worktree Utility Commands) of the workspace improvement plan:

• gx workspace root — prints the main worktree root on stdout, clean for scripts and aliases like cd "$(gx workspace root)".
• gx workspace move <workspace> <new-path> — relocates a workspace via git worktree move. Refuses the main worktree and an existing destination; follows the shell wrapper into the new path when the current workspace is moved.
• gx workspace lock <workspace> [--reason <reason>] — locks a worktree so cleanup and git worktree prune skip it. Refuses the main worktree.
• gx workspace unlock <workspace> — clears a worktree lock.
• gx workspace repair [workspace] — wraps git worktree repair for moved or damaged worktrees; repairs a single matched worktree, or all of them when no query is given.

Why

These useful git worktree operations were not discoverable through gx even though they fit naturally in the workspace namespace. lock/unlock in particular close a loop: gx already parses and displays locked state, so exposing the commands completes that feature.

How it works

• Git helpers live in src/git/worktree.rs as thin wrappers around the matching git worktree subcommands. Each runs git from the main worktree root (where git requires worktree admin operations to run) and ends option parsing with -- so paths beginning with - are never treated as flags. Argument builders are split into pure functions for unit testing.
• The move and lock/unlock/repair workspace arguments reuse the existing fuzzy workspace resolution (same matching as gx workspace go).
• Safety: move refuses the main worktree and existing destinations; lock refuses the main worktree. Move-destination resolution expands ~ and makes relative paths absolute against the current directory via shared helpers (expand_home, resolve_dest_path), and expand_home is reused to de-duplicate the existing tilde-expansion logic in workspace_path.
• All human-readable output goes to stderr, keeping stdout reserved for shell navigation. The single exception is root, whose entire purpose is to emit a path value on stdout.

Testing

• cargo build — passes, no warnings.
• cargo test — 136 tests pass, 0 failures. New tests cover the argument builders (move/lock with and without reason/unlock/repair forwarding paths) and the path helpers (expand_home, resolve_dest_path).
• Modified files formatted with rustfmt.