Add SYO runtime expected config seed

[cbusillo]

Summary

• Add SYO product onboarding seed material for canonical sellyouroutboard testing/prod lanes.
• Declare SYO live-target-runtime expected runtime keys and managed runtime secret bindings so product-scoped runtime sync can validate the lane instead of failing runtime_environment_empty.
• Preserve sellyouroutboard-testing as a historical context in seed material and make onboarding merge existing historical contexts during re-imports.
• Document that pre-cutover context audit rejects historical contexts after cutover; post-cutover evidence should use legacy cleanup dry-run plus canonical live-runtime validation.

Evidence

• Failed pre-fix SYO testing dry-run: run 26849420093, trace launchplane_req_7bbf3f983a0c4364a35d4b474cdee102, runtime_environment_empty for sellyouroutboard/testing.
• Read-only agent confirmed context audit 403 is expected post-cutover because sellyouroutboard-testing is historical, not current product boundary.
• Agent review found a historical-context overwrite risk; this PR hardens onboarding to merge existing historical contexts instead of replacing them with only manifest values.

Local Validation

• uv run python -m unittest tests.test_product_onboarding tests.test_product_onboarding_service
• uv run python -m unittest
• uv run --extra dev ruff check control_plane/contracts/product_onboarding_manifest.py control_plane/workflows/product_onboarding.py tests/test_product_onboarding.py tests/test_product_onboarding_service.py
• uv run --extra dev ruff format --check control_plane/contracts/product_onboarding_manifest.py control_plane/workflows/product_onboarding.py tests/test_product_onboarding.py tests/test_product_onboarding_service.py
• uv run --extra dev mypy control_plane/contracts/product_onboarding_manifest.py control_plane/workflows/product_onboarding.py tests/test_product_onboarding.py tests/test_product_onboarding_service.py
• markdownlint-cli2 docs/records.md