chore: add ruff linter, fix mypy errors, and write README

- Add ruff linter configuration to pyproject.toml with modern Python 3.12 rules
- Add ruff~=0.9.4 to dev dependencies
- Fix all mypy type errors (Optional[] hints, Token types, Any returns)
- Auto-fix 54 ruff issues (import sorting, Optional -> X | None syntax)
- Create ProjectMeta dataclass for single source of truth from pyproject.toml
- Write comprehensive README.md with setup, config, and development docs
- Update main.py to use settings.app_description from pyproject.toml

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/domains/router.py

@@ -7,6 +7,7 @@ main.py only includes this root_router.
 from fastapi import APIRouter
 
 from src.domains.health.router import router as health_router
+
 # from src.domains.auth.router import router as auth_router
 # from src.domains.agents.router import router as agents_router
 # from src.domains.tools.router import router as tools_router