feat(gitstatus): untracked file cleanup overlay

[jongio]

What this does

Adds a cleanup overlay to the Git Status panel for safely removing untracked files. Press X to open a preview built from git clean dry-run output, multi-select the entries you want gone, then confirm. Only the selected paths are removed. Including ignored files is opt-in and listed separately from plain untracked files.

Closes #272

How it works

flowchart TD
    A[Press X in Git Status] --> B[git clean -nd dry run]
    B --> C{Include ignored?}
    C -- yes --> D[git clean -ndX for ignored]
    C -- no --> E[Untracked only]
    D --> F[List candidates, ignored marked separately]
    E --> F
    F --> G[Multi-select with space]
    G --> H{Confirm removal?}
    H -- no --> I[Close, nothing removed]
    H -- yes --> J[git clean -fd -- selected paths]
    J --> K[Refresh status and file tree]

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Changes

• New internal/git/clean.go: CleanPreview runs git clean -nd (untracked) and, when requested, git clean -ndX (ignored only) so ignored candidates are separated. Clean runs git clean -fd -- <paths> (adds -x when ignored are included). Empty path list is a no-op for safety, and every path is validated with ValidateRepoRelativePath before it reaches git.
• Dry-run output is parsed under a forced C locale so the "Would remove" prefix is stable across languages.
• New cleanup overlay in internal/panels/gitstatus/clean.go: candidate list, multi-select, select-all, cursor, counts, and a confirmation step that reuses the existing destructive-operation prompt.
• Keybinding X registered in the single source of truth (keybindings.json), mirrored in the panel KeyBindings() and regenerated docs/keybindings.md.

Testing

• internal/git: table tests for dry-run parsing plus integration tests against a real repo for preview and remove behavior, including the opt-in ignored path.
• internal/panels/gitstatus: overlay open/close, selection toggles, select-all, confirm dispatch, and rendering.
• go build ./..., go vet ./..., gofumpt -l clean.

Safety

• Nothing is removed without explicit confirmation.
• Removal is scoped to the exact selected paths; no selection means no operation.
• Ignored files require an explicit opt-in and are visually separated from untracked files.
• All paths are validated before being passed to git.