Add semver package artifact workflows

[rickardvh]

Summary

• Add PR CI that runs tests/lint, builds wheel and sdist artifacts, proves wheel installation outside the source tree, and uploads dist/ artifacts.
• Add a semver tag release workflow that validates tag/version alignment, rebuilds and smoke-tests artifacts, then attaches them to a GitHub Release.
• Document the release mechanics and add workflow regression tests so the artifact path stays durable.

Proof

• uv run pytest
• uv run ruff check src tests
• uv build
• built-wheel install smoke from a fresh uv venv outside the source tree
• git diff -- README.md docs .agentic-workspace/docs packages/planning/README.md packages/memory/README.md

Resolves #16