Detect existing gg shell command collisions

[chtnnh]

Summary

• Add gg doctor --shell checks for persisted Bash, Zsh, and Fish gg collisions
• Provide safe shell-specific setup guards that refuse to overwrite an active gg definition
• Cover Fish abbreviations, including regex and command-scoped definitions

Validation

• Added red-to-green regressions for persisted and active shell collisions
• Ran the full local CI gate and documentation build
• Completed two independent diff-only final reviews