Allow workflow specs to be full GitHub links

[Copilot]

Resolves the issue by adding support for full GitHub URLs as workflow specifications in the gh aw add command.

Problem

Previously, users could only add workflows using the short-form specification format:

gh aw add githubnext/gh-aw-trial/workflows/release-issue-linker.md

This required manually constructing the spec string with the correct format. When browsing workflows on GitHub's web interface, users couldn't simply copy the URL and use it directly.

Solution

Users can now use full GitHub URLs directly:

gh aw add https://github.com/githubnext/gh-aw-trial/blob/main/workflows/release-issue-linker.md

The implementation automatically:

• Extracts owner/repo from the URL
• Identifies the file path within the repository
• Detects the branch, tag, or commit SHA from the URL structure
• Validates the URL format and domain

Supported URL Formats

All standard GitHub URL formats are supported:

1. Blob URLs (most common - from GitHub web UI):

   https://github.com/owner/repo/blob/branch/path/to/workflow.md
   

2. Tree URLs (from directory views):

   https://github.com/owner/repo/tree/branch/path/to/workflow.md
   

3. Raw URLs (direct file content):

   https://github.com/owner/repo/raw/branch/path/to/workflow.md
   

All URL formats work with branches, version tags, and commit SHAs.

Implementation Details

• Added parseGitHubURL() function to parse and validate GitHub URLs
• Updated parseWorkflowSpec() to detect HTTP/HTTPS URLs and route them appropriately
• Maintains full backward compatibility with existing spec formats
• Includes comprehensive validation with clear error messages

Testing

• Added 16 new test cases covering all URL formats and error conditions
• All existing tests continue to pass
• Documentation and help text updated with examples

Benefits

• Convenience: Copy URLs directly from GitHub's web interface
• Clarity: Branch/tag information is visible in the URL
• Flexibility: Works alongside all existing spec formats
• Safety: Validates domain, path structure, and file extensions

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Allow workflow specs to be full github links</issue_title>
<issue_description>We should allow workflow specs to be full github links too, e.g.

gh aw add githubnext/gh-aw-trial/workflows/release-issue-linker.md 

could be

gh aw add https://github.com/githubnext/gh-aw-trial/blob/main/workflows/release-issue-linker.md

That gives a branch story as well</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #1259

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