Make Config.Validate() functional or remove it

[Fuabioo]

Problem

The Config.Validate() method always returns nil, making it a no-op.

Evidence

config.go:150-157

func (c *Config) Validate() error {
    if _, err := os.Stat(c.ProjectsDir); os.IsNotExist(err) {
        return nil  // Not an error - returns nil even when dir doesn't exist
    }
    return nil  // Always nil
}

The method is named Validate but doesn't validate anything - it always returns nil regardless of input.

Impact

• Misleading API - callers expect validation to occur
• Future maintainers might add calls to Validate() expecting it to work
• Defensive programming relies on validation actually validating

Suggested Fix

Either:

1. Make it validate something useful, such as:
   • Check if ClaudeBin exists and is executable
   • Verify DBPath parent directory is writable
   • Validate Backend is a known value
2. Remove the method entirely if validation isn't needed

Files: internal/config/config.go:150-157