Migrate worktree-tidy onto core run_pipeline scan seam

[jakebromberg]

Summary

Migrates git-worktree-tidy onto the shared git_tidy_core::scan::run_pipeline seam with ScanOptions { fetch: true }, and deletes the now-dead worktree-specific core types. worktree-tidy was the last tool still using the non-generic core::types::{RepoGroup, ScanResult} that PR B's generics were modeled on, so this PR also shrinks core::types for the first time.

What changed

• run_scan_repos routes through run_pipeline. The per-repo git fetch --prune is handed to the seam (the last fetch: true tool), dropping the tool's manual parallel_fetch, parallel_classify, empty-group check, and counts/total_scanned fold. The closure now returns the repo's Vec<WorktreeInfo> (empty for skipped repos); the seam wraps non-empty groups, folds Counts by Classified::classification_label, and sums total_scanned.
• Discovery and the --match filter stay in the thin run_scan, applied to the parent-repo map before the pipeline. The closure still captures that map and looks worktrees up by parent repo — the documented worktree deviation (parent-repo paths in, worktree map captured).
• Core-type deletion + orphan-rule inversion. Removes the non-generic RepoGroup, ScanResult, and their FlatJsonItems impl from core::types. Because WorktreeInfo is a core type, its new Classified and IntoJsonItem impls live in core (the inverse of branch-tidy, where BranchInfo is a tool-crate type). A WorktreeScanResult = ScanResult<WorktreeInfo> alias keeps a stable type name for the tool and the audit runner, preserving repos/total_scanned/counts/warnings.

Behavior

Public output is byte-identical: flat --json array, 9-field porcelain (worktree dir then parent repo, via the untouched TidyItem impl), and the LANDED_SUMMARY human line. Item ordering is preserved by the closure's sort_by_key on classification priority; fetch parity is one git fetch --prune per repo as before. The audit runner is untouched (run_scan 8-arg signature and |r| &r.counts unchanged).

Tests

All existing scan/output/clean/discovery tests pass unchanged save the worktrees→items / ScanResult→WorktreeScanResult fixture renames. Adds run_scan_repos_drops_repo_when_default_branch_errors, a regression guard confirming a repo present in the map with a worktree but with failing default-branch detection yields no group and surfaces the warning — the empty-group drop the old manual is_empty() check provided, now provided by the seam.

Local gate jobs green: cargo fmt --check, cargo clippy --workspace --all-targets -D clippy::all, cargo test --workspace, cargo +1.93.0 check --workspace (MSRV).

Part of #117