docs: clarify set-mode mode ids are adapter-defined

[z-x-yang]

Summary

This docs update clarifies that set-mode values are adapter-defined and not guaranteed to be portable across all ACP adapters.

Changes

• README.md
  • Replace acpx codex set-mode plan example with acpx codex set-mode auto.
  • Add note that unsupported mode ids are rejected by adapters (commonly Invalid params).
• docs/CLI.md
  • Add behavior notes under set-mode explaining adapter-defined mode ids and rejection semantics.
• skills/acpx/SKILL.md
  • Align example and behavior note with the same clarification.

Why

Users can copy examples like set-mode plan and reasonably expect universal behavior. In reality, whether plan is accepted depends on the selected adapter's exposed modes.

This keeps examples practical and reduces confusion without changing runtime behavior.