Download snapshots from GitHub

[hadley]

Don't see any obvious way to test this, but I tried it out a bunch interactively. Snapshot test deliberately failing so I can double check that the message is useful.

Fixes #1779

[claude]

Claude finished @hadley's task —— View job

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PR Review: Download snapshots from GitHub

Reviewed the snapshot download functionality for potential bugs and issues.

Issues Found

High Priority:

1. No input validation for run_id parameter (R/snapshot-github.R:18): The function accepts run_id as a number but GitHub run IDs are strings that can overflow numeric types. Should use check_string() instead of check_number_whole().

2. Unsafe regex parsing (R/snapshot-github.R:82): The regex "Artifact download URL: (?<artifact_url>.*)" uses .* which could match malicious URLs. Should validate URL format or use more restrictive pattern.

Medium Priority:
3. No error handling for GitHub API calls (R/snapshot-github.R:55,74,96): All gh::gh() calls lack try-catch blocks. Network failures or auth issues will crash with unhelpful errors.

4. Potential directory traversal (R/snapshot-github.R:39,40): No validation that src_snaps path is safe before copying. Malicious artifacts could contain ../../../ paths.

5. Overwrites files without confirmation (R/snapshot-github.R:40,138): dir_copy() and file.copy() overwrite existing files. Should warn users about overwrites.

Low Priority:
6. Missing dependency check (R/snapshot-github.R:21): Only checks for gh package but also uses withr, cli, and utils functions that could fail.

7. No tests for new functionality: The main snapshot_download_gh() function has no direct tests, only integration via the hint system.

Minor:
8. Inconsistent variable naming (R/snapshot-github.R:34-38): Mixed use of files vs inner_dir could be clearer.

The deliberately failing test mentioned in the PR description appears to be the snapshot hint integration test, which correctly shows the new download command format.