fix: stage removed worktrees in .git/wt-trash/ instead of sibling path

[worktrunk-bot]

Summary

• Moves the rename-based staging directory from a visible sibling path (project.wt-removing-<timestamp>) into .git/wt-trash/, hiding it from the user's workspace
• Adds Repository::wt_trash_dir() accessor following the existing wt_logs_dir() pattern
• Falls back to legacy git worktree remove if the trash directory can't be created
• The .git/ directory is always on the same filesystem as worktrees, so the instant rename guarantee is preserved

Context

Users reported confusion when seeing .wt-removing-* directories in their workspace after wt remove (#1572). By staging in .git/wt-trash/ instead, the directory is completely hidden — even if the background rm -rf is slow or gets interrupted.

Test plan

• Unit tests for generate_removing_path and build_remove_command_staged updated and passing
• All 105 remove-related integration tests passing
• test_remove_background_path_gone_immediately — verifies instant removal still works
• test_remove_background_fallback_on_rename_failure — verifies fallback when staging path is blocked
• test_remove_stale_staging_dir_from_crashed_removal — verifies stale dirs land inside .git/

Closes #1572

🤖 Generated with Claude Code

[worktrunk-bot]

The doc comment on generate_removing_path claims .git/ is always on the same filesystem as the worktree, but that's only guaranteed for the main worktree. Linked worktrees can live on different mount points — in which case rename() fails with EXDEV and the fallback handles it correctly. The comment should reflect this.

[max-sixty]

what other paths in .git do we write to?

[worktrunk-bot]

We currently write to four paths under .git/:

Path	Purpose
wt-logs/	Post-start command logs, verbose logs (-vv), diagnostic reports
wt-cache/summaries	Cached worktree summaries for wt list
wt-cache/ci-status	CI status cache for wt list
wt-promote-staging/	Temporary staging for wt step promote
wt-relocate-tmp/	Temporary staging for wt relocate

And this PR adds wt-trash/ as the sixth. All follow the same pattern — git_common_dir().join("wt-<name>") so they're centralized in the main worktree's .git/ and shared across linked worktrees.

[max-sixty]

is this standard?

should we have a .git/wt path?

[worktrunk-bot]

Good idea — we've accumulated six wt-* siblings in .git/:

Current	Proposed
wt-logs/	wt/logs/
wt-cache/summaries/	wt/cache/summaries/
wt-cache/ci-status/	wt/cache/ci-status/
wt-promote-staging/	wt/promote-staging/
wt-relocate-tmp/	wt/relocate-tmp/
wt-trash/ (this PR)	wt/trash/

A single .git/wt/ parent would be tidier. Happy to do that consolidation here or as a follow-up — the rename is mechanical but touches ~15 files. What's your preference?