feat: define Mango-native HTTP transport contract

[yanpgwang]

What changed

• require Authorization: Bearer <workspace-key> on protected Mango routes and stop accepting inbound x-api-key
• remove mango serve -strict plus Anthropic version/beta middleware
• enforce documented JSON or multipart content types for every non-empty request body
• keep the generic request-id response contract
• replace Anthropic-Worker-ID with the optional worker_id polling query parameter
• update OpenAPI, API/security/architecture/capability docs, raw HTTP tests, and SDK research fixtures

Closes #165.

Verification

• make verify
• make docs-check
• make security
• make local-config
• make image-smoke
• git diff --check

API impact and design provenance

This intentionally changes the pre-release /v1 transport contract in place. Clients must use bearer authentication; no compatibility alias, /v2, deprecation window, or strict-mode dual behavior is retained. Environment Work clients pass operational correlation as worker_id; it is not a credential.

Claude Managed Agents documentation and public SDK behavior informed the former authentication, rollout, and worker header choices. Mango adopts standard bearer/content-type semantics and retains the sound generic request correlation shape, while rejecting provider rollout headers and provider-named worker correlation. The decision is recorded in docs/provenance.md. Anthropic headers remain only in the outbound Messages adapter.

Durability and security

No persistence, Session lifecycle, workflow, retry, or side-effect semantics change. Authentication still resolves exactly one Workspace before a protected handler runs. Content-type validation runs before request decoding, and request IDs remain available on error paths. There is no data migration; pre-release clients must update their request headers and Environment Work poll URL.

Checklist

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