Tweaks to docs, gh aw help and notice

[dsyme]

Documentation improvements:

• Updated all documentation warnings to use [!WARNING] and clarified that the extension is a research demonstrator in early development. [1] [2] [3]
• Expanded documentation to clarify the difference between Claude (default) and Codex (experimental) engines, including their required secrets and MCP support. Added explicit instructions and examples for engine selection, secret setup, and environment variable usage in workflow frontmatter. [1] [2] [3] [4] [5]
• Improved documentation on MCP server secrets, custom environment variables, and security best practices.

Package management and CLI features:

• Added documentation and examples for new package removal commands (gh aw uninstall org/repo), distinguishing between global and local removal, and described related features. [1] [2]
• Updated CLI output to mark packages as "local" or "global" with a comma for clarity.

Command-line usability enhancements:

• Overrode the help function in main.go to hide the completion command from help output for a cleaner user experience.
• Documented new gh aw compile flags for engine override, schema validation, and Copilot instructions generation.

Minor code and documentation fixes:

• Clarified the context text variable in workflow frontmatter documentation.
• Updated comments to reflect that workflows can be run if they have schedule or workflow_dispatch triggers, not just locally.
• Removed redundant engine compatibility section from tools documentation.