GitHub fetching and caching

[jimmytacks]

Part of #1

Description

Fetch LAMBDA libraries from GitHub repos at runtime and cache locally.

Deliverables

• GitHubSource.cs — Fetch library contents from raw.githubusercontent.com. Discover libraries via GitHub API (GET /repos/{owner}/{repo}/contents/lambdas), fetch files via raw URLs
• SourceCache.cs — Cache to %LOCALAPPDATA%\LambdaBoss\cache\{repo-hash}\. Serve from cache when available, invalidate on explicit refresh
• RepoConfig.cs — Model for configured repo source (URL, enabled/disabled, last-fetched timestamp)

Fetching strategy

1. Directory listing: GitHub API (/repos/{owner}/{repo}/contents/lambdas) — 1 request per repo
2. Library metadata + LAMBDA files: raw.githubusercontent.com (no rate limit)

Tests

• Unit tests for GitHubSource with mocked HTTP responses
• Unit tests for SourceCache (write/read/invalidate)
• AddIn test: configure this repo as a source, fetch and load a test library

Depends on

• LAMBDA file parsing and prefix rewriting #3 (LAMBDA file parsing)