feat: add subpath support for monorepo sparse checkout

[ChrisLally]

Description

Fixes #27

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.

^ For example, I only wanted part of the vercel repo so the command "opensrc vercel/vercel/packages/cli" worked great now!

Supported syntax

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

Implementation

• Parsing (lib/repo.ts): Extended parseRepoSpec to handle 3+ path segments and extract a subpath (e.g. vercel/ai/packages/ai → owner: vercel, repo: ai, subpath: packages/ai).
• Types (types.ts): Added optional subpath to RepoSpec and ResolvedRepo.
• Fetch (lib/git.ts): When subpath is set, uses git clone --filter=blob:none --sparse and git sparse-checkout set <subpath> instead of a full clone.
• Output path: Files are stored at opensrc/repos/github.com/owner/repo/packages/ai to mirror the repo layout.

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

• Added tests for subpath parsing in parseRepoSpec (owner/repo/subpath, URLs with tree/ref/subpath, github: prefix).
• Added tests for isRepoSpec and detectInputType with subpath formats.
• Manually tested with opensrc 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]

Additional Suggestion:

The "already up to date" early return in fetchRepoInput omits the subpath from the returned package and path fields, and looks up repo info using the base name instead of the subpath-qualified name.