docs: publish the composable stack guide

[leynier]

Summary

• replace the legacy README with the v0.5 layer, preset, deploy, and skills model
• add root Zensical documentation and a strict documentation CI job
• configure Vercel static builds for the root site at python-template.leynier.dev
• modernize contribution guidance and issue forms for composable workloads

Validation

• uv run pytest -n auto (83 passed)
• uv run ruff check .
• uv run ruff format --check .
• uv run python scripts/compile_catalog.py --check
• uv run --only-group docs zensical build --clean --strict
• gh act pull_request -j lint --pull=false
• gh act pull_request -j docs --pull=false

Deployment note

The Vercel CLI is installed but this machine has no authenticated Vercel session or VERCEL_TOKEN. The repository config is ready; project creation and GitHub linking will follow once authentication is available.