install: stage git cache folders and rename on success; hit only tagged checkouts - #38274

`delete_file_z` is `unlinkat(.., 0)` (no `AT_REMOVEDIR`), so a repository that checks in a **directory** named `.bun-tag` survives the unlink and then `O_CREAT|O_EXCL` fails EEXIST — the install hard-fails with `writing ".bun-tag" failed: EEXIST`, whereas before this PR it installed (with degraded verify). `dir.delete_tree(b".bun-tag")` (matching `.git` two lines up) would make the comment "anything the repository checked in under that name is replaced" true; alternatively the error could name t

nit: `CacheStaging` names a temp directory that must be removed on every error path, and the six error returns between `CacheStaging::new()` and `publish()` each manually call `staging.discard()`. All current paths do so correctly, but per CLAUDE.md #8 ("Prefer RAII (`Drop`) over manual cleanup") a `Drop` impl that calls `discard()`, disarmed by a `published: bool` set in `publish()`, would collapse those calls and stay correct if a future `?` or early return is added. Not blocking — worst-case 

nit: the three success-path checks (`expect(await install()).toMatchObject({ exitCode: 0 })`, also 5444/5451) assert only `exitCode`. `toMatchObject`'s failure diff does print the full received object so `err` is not actually hidden, but destructuring and asserting `err` first — as the failure path at 5438-5440 already does — would surface git's stderr as the primary diff line rather than buried in the `+ Received` block. Worth it here since the PR description defers Windows rename behavior to C