Remove stale v0.9.0 release-branch context

[xeonvs]

Goal

Remove the final stale post-release statement discovered during the full v0.9.0 completion audit. docs/engineering/m5_context_contracts.md still claimed that the implementation lived on an active release branch and that remaining stable-release limits lived in an active plan, although v0.9.0 is published, externally reconciled, and all release branches are deleted.

This is a no-release documentation correction. It does not modify runtime, package versions, OCR compatibility, workflows, release objects, or artifacts.

Plan

1. Replace ephemeral active-branch/active-plan ownership with durable main, test-evidence matrix, and execution-history ownership.
2. Enforce the wording through the existing documentation ownership test rather than adding a new test surface.
3. Run focused formatting, lint, documentation contracts, diff validation, and history secret scanning.
4. Merge through the protected path, delete the branch, synchronize main, and read back the normal development workflow outcome.