docs(release): prepare v0.6.0

[typelicious]

What changed

• bump package and app version from 0.5.0 to 0.6.0
• move the current Unreleased notes into a v0.6.0 changelog section
• update release documentation and examples for the v0.6.0 tag

Why

The v0.6.x modality-expansion work is now on main, so the repo is ready for a clean v0.6.0 release cut.

How verified

• PYTHONPYCACHEPREFIX="$PWD/.pycache" python3 -m compileall foundrygate tests
• PYTHONPATH=. ./.venv-check-313/bin/pytest -q
• ./.venv-check-313/bin/ruff check .
• ./.venv-check-313/bin/ruff format --check .
• ./.venv-check-313/bin/python -m build --no-isolation
• ./.venv-check-313/bin/python -m twine check dist/foundrygate-0.6.0*
• /usr/bin/git diff --check