MM-14: Contract legacy provider-visible rewrite paths

Parent

#771

Checkpoint

MC-3 ContextEngine ownership

What to build

Remove or seal legacy paths that can rewrite provider-visible messages outside ContextEngine and prevent double compaction.

Acceptance criteria

• All supported surfaces invoke one assembly owner.
• No model-visible rewrite occurs after ContextEngine validation.
• Double compact is detected and rejected.
• Static/runtime contract tests cover forbidden bypasses.
• Compatibility strategy still exists inside ContextEngine.

Required tests

• Call-site contract
• double compact
• provider validation
• all surfaces
• unknown legacy hook

Benchmark

Repeat MC-3 equivalence and general-task pilot after contract.

Blocked by

• [MM-12] Migrate headless context assembly to ContextEngine #791 (MM-12)
• [MM-13] Migrate desktop and CLI context assembly to ContextEngine #792 (MM-13)

Rollout and rollback

Re-enable only the compatibility strategy, not external rewrite ownership.