Add agent skills, sandboxed templates, fix AGENTS.md violations

[tedivm]

Summary

• Created 10 agent skills for domain-specific guidance (makefile, pydantic-settings, python-testing, typer-cli, fastapi-routes, sqlalchemy-models, docker-compose, celery-tasks, aiocache, jinja-templates)
• Reduced AGENTS.md from 425 → ~250 lines by moving detailed guidance to skills
• Added Jinja2 sandboxed environment support with SandboxedEnvironment and 9 security tests
• Replaced print() with logger.info() in celery.py and qq.py
• Added .env.example file and /health endpoint

Changes

Agent Skills (10 new):

• makefile — complete reference for all make targets
• pydantic-settings — configuration settings conventions
• python-testing — pytest patterns and fixture structure
• typer-cli — CLI commands and @syncify pattern
• fastapi-routes — REST API endpoints and Pydantic models
• sqlalchemy-models — async SQLAlchemy models and Alembic migrations
• docker-compose — development environment workflows
• celery-tasks — background tasks and periodic task setup
• aiocache — caching with @cached decorator and NoOpCache
• jinja-templates — template rendering including sandboxed environments

AGENTS.md Reduction:

• Shrunk from 425 to ~250 lines
• Replaced detailed feature sections with skill cross-references
• Kept global best practices (General, Security, Production Ready, Logging, etc.)

Code Quality Fixes:

• celery.py, qq.py: print() → logger.info() (AGENTS.md compliance)
• test_celery.py, test_qq.py: capsys → caplog with set_level(logging.INFO)
• www.py: added /health endpoint returning {"status": "ok"}
• Added .env.example with all conditional env vars commented out

Sandboxed Templates:

• Added sandbox_env (SandboxedEnvironment) in services/jinja.py
• Created jinja-templates/references/sandboxed-templates.md documentation
• Added 9 sandbox security tests (chain attacks, dunder blocking, etc.)

All 3 Configurations Pass:

• bare: 11 tests, 100% coverage
• library: 26 tests, 93% coverage
• full: 157 tests, 94% coverage
• All mypy clean, ruff clean, black clean