refactor: establish SUPPORTED_AGENTS as single source of truth

[arittr]

• Export SUPPORTED_AGENTS from agents/types.ts as canonical agent list
• Update agentNameSchema to derive from SUPPORTED_AGENTS (removes hardcoded duplication)
• Add comprehensive tests verifying schema-constant synchronization
• Remove AgentName type export from types/schemas.ts to prevent confusion
• Format code with updated linter rules

🤖 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.}