Docs: define ACP-only JSON stream contract

[dutifulbob]

Summary

This PR updates ACPX docs to make the JSON stream contract ACP-only.

Key changes:

• docs/2026-02-27-acpx-session-model.md

  • Rewritten to specify that the canonical stream is raw ACP JSON-RPC only.
  • Removes custom stream envelope/event-schema guidance.
  • Clarifies boundary: local runtime/bookkeeping state stays out-of-band.

• docs/CLI.md

  • Updates --format json semantics to raw ACP NDJSON stream.
  • Removes acpx.event.v1 envelope examples and synthetic control-event mapping.
  • Adds explicit ACP-stream hard rules (no wrapper fields, no key renaming).

• docs/2026-02-22-openclaw-integration-plan.md

  • Aligns integration plan wording with ACP-only stream direction.
  • Replaces old envelope-oriented acceptance criteria with ACP JSON-RPC criteria.

Why

To enforce a single protocol surface in streamed automation output:

• ACP stream payloads should remain ACP, unchanged.
• acpx-specific orchestration metadata should not be injected into the ACP stream.
• This reduces schema drift and simplifies downstream orchestrators (including OpenClaw).

Scope

Docs-only. No runtime behavior changes in this PR.