Promote ask_knowledge_base as RAG Sub-Agent in AGENTS_STARTUP.md

We need to update AGENTS_STARTUP.md to explicitly promote the ask_knowledge_base tool as a RAG Sub-Agent.

Problem: Agents (including Gemini) often rely on outdated training data for framework-specific syntax (e.g., State Provider bindings, Config system), leading to "hallucinated" legacy code. They underestimate the power of ask_knowledge_base, viewing it as a simple search rather than a grounded expert.

Solution: Refactor AGENTS_STARTUP.md to include a new "Ask the Expert" Protocol.

1. Rebrand the Tool: Explicitly define ask_knowledge_base as an "Embedded RAG Sub-Agent" that has read-access to the current repository state.
2. Mandate Usage for Syntax: Enforce a rule that agents MUST use this tool when generating code for core framework features (Bindings, Configs, Workers, VDOM) to verify current syntax.
3. Explain the "Why": Highlight that this tool performs a "Read-Eval-Generate" loop: it retrieves relevant files, reads their full content, and synthesizes an answer based only on the current codebase, making it the single source of truth for syntax.
4. Provide Examples: Show concrete queries like:
   • ask_knowledge_base(query='current syntax for state provider bindings')
   • ask_knowledge_base(query='how to implement a reactive config')

This will shift agent behavior from "guessing based on training" to "verifying against the repo".