feat(shell): complete cd -jw with native shell candidates #40

Parent

Part of #43.

Summary

Refine worktree Tab completion so the shell receives candidate paths instead of cj launching an interactive fzf process from inside completion.

Interaction

1. Type cd -jw or cd --jump-worktree.
2. Press Tab.
3. The generated integration asks cj for this repository's worktree paths.
4. The shell presents those candidates using its normal completion UI.
5. The chosen worktree safely replaces the jump token; Enter performs the directory change.

With fzf-tab or another completion frontend installed, that frontend can render the worktree candidates through fzf. Without fzf, native completion still works.

Acceptance criteria

• included by cj init <shell> without --setup-key-binding
• support Bash, Zsh, Nushell, and PowerShell
• return only worktree path candidates for an exact current -jw or --jump-worktree token
• do not launch a nested fzf process during Tab completion
• preserve exact paths and shell-safe insertion for spaces, apostrophes, Unicode, leading dashes, platform-native separators, and non-UTF-8 Unix paths where the shell permits them
• cancellation or an empty candidate set leaves the buffer and current directory unchanged
• ordinary cd completion delegates to the existing shell-specific behavior
• Zsh registration remains effective when compinit and fzf-tab/plugin frameworks initialize before or after the generated integration
• cj -jw and cd -jw<Enter> remain explicit fzf picker paths and retain useful missing-fzf diagnostics
• standalone cj completions behavior remains separate; wrapper completion stays in generated init
• native, generated-source, external integration, and Windows CI tests are green

Scope note

This supersedes the original direct-fzf-on-Tab wording after interactive testing exposed nested/ordinary completion behavior. Reuse the existing shell hooks, quoting, and tests from PR #41 where they still fit.