fix: Respect gitignore without git metadata (#13756)
## Why
`turbo prune --use-gitignore` did not honor `.gitignore` files when Git
metadata was absent, as in Docker build contexts. This could copy
workspace-local `node_modules` into `out/full` and leave stale or
dangling package-manager links.
Fixes #13755.
## What
- Configure the shared `CopyPlan` walker with `require_git(false)` so
enabled gitignore matching does not depend on a `.git` directory.
- Add focused filesystem coverage for a planned copy without Git
metadata.
- Exercise `prune --docker --use-gitignore` after removing `.git` and
verify workspace `node_modules` remains excluded.
## Testing
- `cargo fmt --all -- --check`
- `cargo test -p turborepo-fs`
- `cargo test -p turbo --test prune_test
test_prune_respects_root_gitignore_in_workspaces`
- `cargo clippy -p turborepo-fs -p turbo --all-targets -- -D warnings`
- `cargo test -p turbo --test prune_test` (25/26 pass; pre-existing
`test_prune_composable_config` fails during `pnpm install`, matching the
known environment failure documented in #13705)
Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
