Allow Launchplane payloads to define publish contexts

[cbusillo]

Summary

• synthesize missing runtime contexts during artifact publish only when an explicit Launchplane/devkit runtime payload is configured
• keep unknown contexts fail-closed without that payload
• document the Launchplane payload publish behavior

Validation

• uv run python -m unittest discover -s tests
• uv run ruff check odoo_devkit/local_runtime.py tests/test_runtime.py
• uv run ruff format --check odoo_devkit/local_runtime.py tests/test_runtime.py
• Claude pre-merge review: no blockers