Add Git-aware clean mode with grouped candidates

[Byron]

Tasks

This section is for Byron only. Models continuing this PR must not add, remove, check, uncheck, rename, or reorder checkboxes here.

• refackiew

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Everything below this line was generated by Codex.

Created by Codex on behalf of Byron. Byron will review before this is ready to merge.

Changes

dua clean [DIRECTORY]... finds disposable directories, sizes them, and opens the existing TUI. Multiple sibling candidates share a virtual parent; deeper candidates join their nearest containing group, keeping a Python project's caches together. Group sizes and counts include only the candidates. Marking or deleting a group operates on its candidates and preserves the parent and ordinary source files. Single candidates outside groups remain separate roots.

Discovery reads directory entries/types before detailed size scans and prunes matched artifacts and Git internals. Supported names cover Node dependencies, Python caches and virtual environments, Cargo build output, and Zig's .zig-cache, legacy zig-cache, and zig-out. Cargo and virtualenv candidates require project markers. Within Git worktrees, candidates must be ignored and wholly untracked, including conflict stages and gitlink/sparse ancestors. Failed Git checks, nested repositories, unreadable contents, traversal exclusions, and filesystem boundaries reject the candidate.

Group refresh revalidates recorded members using their original ignore-pattern roots without discovering siblings or scanning the real parent. Navigation is restored and groups that lose their siblings are flattened. Full refresh repeats discovery in the original search directories. Marks are cleared before indices are reused. The command supports --depth, traversal options, --once, and --no-entry-check; snapshot import/export and parent scanning are unavailable.

Discovery submits new candidates to a running walker that shares the existing thread budget. Candidates are sized concurrently and published independently, allowing a fast cache to appear while an earlier, slower cache is still scanning. Workers check new roots between directory jobs; the pool stays alive through discovery gaps and finishes after submissions close and all roots complete. The single-thread path remains sequential, and at most 32 candidates are active.

Each candidate has a separate staging tree. Shared hardlink/APFS accounting is committed only after validation, so rejected candidates cannot consume or undo another candidate's accounting. Closing submissions does not block on a full output channel, and walker shutdown disconnects that channel before joining workers.

Active scans also redraw once a second during gaps in traversal events, keeping counters and visible rows current without keyboard input. Refreshing from inside a candidate revalidates the whole owning candidate and restores the current directory if it remains safe.

Discovery and candidate scans conservatively protect ASCII case variants of Git markers, including .GIT gitfiles pointing outside the search directory. The regression covers lower, upper, and mixed-case spellings. Git reference: 1630431f32, t/t0001-init.sh separate-git-dir cases.

Validation

• Deterministic regression holds one root blocked while a newly submitted root completes with the same two workers. Further submissions work after the pool becomes idle.
• Scheduling, full-output cancellation, 40 candidates with 1/2/4 threads, and interleaved hardlink/APFS acceptance/rejection regressions pass.
• Linux and Windows cross-compilation pass.
• A regression holds the input and traversal channels quiet and verifies that counters, rows, and the terminal buffer update during the scan. A live terminal scan also produced output in every second without keyboard input.
• Interior refresh regressions cover both r and R at multiple depths, including a newly added Git marker outside the selected subtree.
• Regression tests first reproduced missing discovery/publication, unsafe tracked candidates, conflict-stage/gitlink gaps, changing ignore scope on refresh, walker shutdown, grouping containment, and missing Zig cache discovery.
• Cleanup journeys cover nested grouping, aggregate sizes, partial/full mark toggles, actual deletion with source preservation, similarly prefixed projects, and refresh without widening scope.
• Zig fixtures cover current and legacy cache names, build output, subtree pruning, ignored caches, unignored directories, and tracked output.
• make check-pre-push: feature combinations, workspace tests, formatting, Clippy, and stateless journeys.
• cargo clippy --workspace --all-targets --all-features -- -D warnings.
• Read-only PTY smoke: Ghostty's .zig-cache and zig-out appeared together
  beneath a virtual parent with two candidates.
• Snapshot fixtures initialize the new runtime grouping flag; the wire format is unchanged.

Git behavior reference: 1630431f32 (v2.55.GIT), Documentation/git-clean.adoc, and t/t7300-clean.sh cases for ignored tracked directories, nested worktrees, and submodules. Gitoxide's gitoxide-core/src/organize.rs informed repository-boundary pruning. Ghostty's build layout and ignore patterns informed Zig support.

Reported issue

$issue-full-auto Add a `dua clean` subcommand which similar to https://github.com/ByteAtATime/oweka provides a TUI that offers directories to delete. Re-use the existing TUI for `interactive` mode, but list only directories that were found using heuristics as roots.
To find these roots, consider using a search that is shallow, and uses well-known directory names, with an initial traversal that doesn't query metadata, just directory entries and types. Once a root is found, it's scheduled for a detailed traversal to learn about file-sizes as well and ultimately be added as root to the interactive tree. `--export` should naturally not be included in `clean`.

Additional user guidance:

And when Git is compiled in and available, double-check that the candidates aren't actually tracked to avoid proposing the deletion of important directories. Maybe also consider ambiguity, i.e. expect deletable items to be gitignored.