docs: clarify CMA surface reuse policy

[yanpgwang]

What changed

• clarify that CMA routes, resource models, JSON shapes, events, and public SDK types are valid Mango design starting points
• make adopted surfaces explicitly Mango-owned without compatibility, synchronization, migration, or release-timing obligations
• align contributor guidance, product documentation, provenance, deployment descriptions, and developer comments
• replace the stale compatibility PR checklist and its nonexistent docs/compatibility.md reference with API-impact and design-provenance guidance

Verification

• git diff --check
• make docs-check

API impact and design provenance

This is a policy and terminology change only. It does not change current HTTP behavior, persisted data, or runtime semantics. Existing vendor-shaped headers and fields remain documented as current implementation facts and should be changed later together with code, OpenAPI, tests, and capability documentation.

The policy now permits deliberate reuse of sound public CMA surfaces while making clear that the adopted contract belongs to Mango and may evolve independently.

Durability and security

No durability, credential, sandbox, or external-call behavior changes.

Checklist

• Tests cover the changed behavior, where applicable.
• Public behavior and policy are documented.
• Material external design influences and Mango-owned decisions are recorded.
• No credentials, generated build output, or local databases are included.