Update documentation - Adding agent via GitHub

[tomkis]

Is your feature request related to a problem? Please describe.
The documentation for adding agents from GitHub repositories is incomplete. It only shows a few basic examples, but the system supports many more URL formats (version tags, branch names, subfolders, enterprise GitHub instances, etc.). Users may not be aware of all supported formats.

Describe the solution you'd like
Update the CLI reference documentation (docs/development/reference/cli-reference.mdx and docs/development/deploy-agents/deploy-your-agents) to comprehensively document all supported GitHub URL formats for the agentstack add command.

Supported formats to document:

• Basic URLs: http://github.com/org/repo, https://github.com/org/repo, git+https://github.com/org/repo
• With .git suffix: https://github.com/org/repo.git
• Version tags: https://github.com/org/repo@1.0.0
• Branch names: https://github.com/org/repo@feature/branch-name
• Subfolder paths: https://github.com/org/repo#path=/a/b.txt
• Combined formats: https://github.com/org/repo.git@1.0.0#path=/a/b.txt
• Enterprise GitHub: https://github.ibm.com/org/repo#path=/a/b.txt
• Dots in names: https://github.com/org.dot/repo.dot.git

Additional context
See agentstack-server/tests/unit/utils/test_github.py for all supported formats and validation rules.