fix: Note and Call_API actions are no-op placeholders

Problem

_handle_note and _handle_call_api in phone_agent/actions/handler.py are no-op placeholders:

def _handle_note(self, action, width, height):
    return ActionResult(True, False)  # placeholder

def _handle_call_api(self, action, width, height):
    return ActionResult(True, False)  # placeholder

The system prompt instructs the model to use Note to record page content and Call_API to summarize:

- do(action="Note", message="True")  
    记录当前页面内容以便后续总结。
- do(action="Call_API", instruction="xxx")  
    总结或评论当前页面或已记录的内容。

But neither action actually does anything. Tasks like "browse multiple restaurant listings and summarize the top recommendations" silently fail.

Proposed Fix

1. Update system prompt so the model writes page content into Note's message field (currently it passes "True" as a static value)
2. Implement Note/Call_API handlers with a lightweight in-memory buffer in agent.py
3. Inject Call_API results into the next step's user message (preserving the user/assistant alternation pattern)

Scope

• 4 files changed: actions/handler.py, agent.py, prompts_zh.py, prompts_en.py
• ~95 lines total, no new files
• No change to the main agent loop architecture
• No extra model calls per step (only Call_API triggers one sub-request)
• Backward compatible: default callbacks are no-ops

I have a detailed implementation plan ready and would like to submit a PR for this. Let me know if this direction makes sense.