P2: Add per-project config file support for unsafe branches

[ericanderson]

We should create a per-project configuration file at ./config/ghouls.json that allows users to:

• Provide their own list of unsafe branches
• Override the default unsafe branch patterns

Implementation Details

• Create config file schema for ./config/ghouls.json
• Support custom unsafe branch patterns
• Allow overriding default unsafe branches
• Document the configuration options
• Add validation for config file