docs: update documentation to reflect Gemini agent support

[arittr]

• Add Gemini to list of supported AI agents in README
• Update architecture.md with simple factory pattern using ts-pattern
• Document factory criteria: single responsibility, pure function, exhaustiveness checking
• Replace "if/else for agent selection" with "simple factory with ts-pattern"
• Update v3 evolution notes to clarify allowed vs banned factory patterns
• Add factory.ts to agent sub-components list
• Update meta.md version history with v3 factory simplification

🤖 Generated with Claude via commitment

[arittr]

This change is part of the following stack:

• refactor(agents): extract commit message cleaning logic to shared utilities #58
  • feat: add agent factory with ts-pattern for type-safe instantiation #59
    • feat: implement Gemini agent with CLI integration #60
      • feat: add Gemini agent support to evaluation system #61
        • refactor: establish SUPPORTED_AGENTS as single source of truth #62
          • docs: update documentation to reflect Gemini agent support #63 ◀
            • feat: add agent configuration support to init command #64
              • fix: preserve staged changes after pre-commit hook #65

_{Change managed by git-spice.}