feat: add subpath support for monorepo sparse checkout

[ChrisLally]

Description

Fixes #27

Ports the sparse-checkout and monorepo subpath behavior from vercel-labs/opensrc#28 to the Rust CLI.

Summary

Adds support for fetching only a subdirectory from a GitHub (or other git host) monorepo instead of the full repo. Uses git sparse-checkout with --filter=blob:none so only the requested path is downloaded.

^ Screenshot: opensrc path vercel/ai/packages/ai — cache directory ~/.opensrc/repos/github.com/vercel/ai/main with only the packages/ai tree checked out.

Supported syntax

opensrc path owner/repo/path/to/dir
opensrc path github:owner/repo/path/to/dir
opensrc path owner/repo/packages/pkg@main
opensrc path https://github.com/owner/repo/tree/main/packages/some-package
opensrc path vercel/ai/packages/ai

Implementation

• Parsing (core/registries/repo.rs): RepoSpec / ResolvedRepo carry an optional subpath; repo specs with three or more path segments (and URL/tree forms) populate it.
• Fetch (core/git.rs): When a subpath is present, clone uses --filter=blob:none --sparse and git sparse-checkout set <subpath> instead of a full checkout.
• Cache (core/cache.rs): Repos live under ~/.opensrc/repos/<host>/<owner>/<repo>/<ref>/ (or $OPENSRC_HOME/repos/... if OPENSRC_HOME is set); the working tree only contains the sparse path.
• CLI (commands/path.rs, commands/remove.rs): opensrc path resolves and prints the absolute directory; remove / sources.json handling understands repo specs with subpaths.

Why sparse checkout

• Avoids cloning large monorepos when only one package is needed.
• Uses Git’s built-in sparse checkout instead of many GitHub API calls.
• Works with any git host (GitHub, GitLab, Bitbucket).

Testing

• Unit tests in core/registries/repo.rs (and related) for subpath parsing across owner/repo/..., github:, and URL/tree forms.
• cargo test --manifest-path packages/opensrc/cli/Cargo.toml from the repo root.
• Manually tested with e.g. opensrc path vercel/ai/packages/ai.

Breaking changes

None. Existing owner/repo usage is unchanged; subpath support is additive.

[vercel]

@ChrisLally is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[vercel]

Suggested change

	if resolved.subpath.is_some() && repo_path.exists() {
	let _ = fs::remove_dir_all(&repo_path);
	if let Some(ref sp) = resolved.subpath {
	if repo_path.exists() {
	// If the requested subpath already exists on disk, reuse it instead
	// of deleting the entire repo directory (which would destroy files
	// checked out for other subpaths of the same repo/ref).
	if repo_path.join(sp).exists() {
	return FetchResult {
	package: format!("{}/{}", resolved.display_name, sp),
	version: resolved.git_ref.clone(),
	path: format!(
	"{}/{}",
	get_repo_relative_path(&resolved.display_name, &resolved.git_ref),
	sp
	),
	success: true,
	error: None,
	registry: None,
	};
	}
	// Subpath not present in the existing clone — must re-clone.
	// This will remove files from any previously sparse-checked-out
	// subpath; callers should handle stale sources.json entries.
	let _ = fs::remove_dir_all(&repo_path);
	}

Fetching a different subpath of the same repo at the same ref deletes and re-clones the shared directory, destroying previously fetched subpath files while their sources.json entries remain as dangling references.