Use git-pkgs clone for HTTPS remotes

[andrew]

Routes HTTPS repositories through github.com/git-pkgs/clone v0.5.0 while retaining the existing git command path for SSH-style sources. Maps depth zero to full clones, keeps positive depths shallow, and adds an optional persistent cache for scan, enrich, and outline. Closes #132.

[Copilot]

Pull request overview

This PR updates remote source resolution to use github.com/git-pkgs/clone for HTTPS remotes (with retries, safer URL handling, and optional caching) while retaining direct git clone for non-HTTPS sources, and wires a new --cache flag through the CLI.

Changes:

• Route https:// clones through git-pkgs/clone (Ensure) and add an optional persistent cache (Cache.Prepare).
• Add --cache flag to scan, enrich, and outline, and plumb it into remote.Options.
• Add unit tests covering HTTPS/non-HTTPS routing, depth mapping, cache behavior, and cleanup on failures.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
remote/remote.go	Switch HTTPS cloning to git-pkgs/clone, introduce cache support, keep git clone for non-HTTPS.
remote/remote_test.go	Add tests for Resolve/clone behaviors (HTTPS shallow/full mapping, cache, cleanup, non-HTTPS fallback).
README.md	Document new --cache flag for remote sources.
cmd/brief/main.go	Add --cache flag to scan and pass through to remote.Resolve.
cmd/brief/enrich.go	Add --cache flag to enrich and pass through to remote.Resolve.
cmd/brief/outline.go	Add --cache flag to outline and pass through to remote.Resolve.
go.mod	Add github.com/git-pkgs/clone v0.5.0 dependency.
go.sum	Add checksums for github.com/git-pkgs/clone v0.5.0.

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