AI: Update Quick Start Guide for Pre-Built Knowledge Base

Context: The migration to gemini-embedding-001 drastically changed the initial setup requirements due to strict daily rate limits. We have implemented a "Zero-Setup" strategy where the Knowledge Base is downloaded automatically via npm install (using the prepare script).

Problem: .github/AI_QUICK_START.md is outdated and misleading:

1. It claims the "free tier is generous" (it's now very restrictive for full syncs).
2. It states embedding "takes 2-5 minutes" (it now takes ~25 mins if done manually).
3. It instructs users to rely on automatic embedding via MCP servers, which we want to avoid for the initial run.

Objective: Rewrite .github/AI_QUICK_START.md to reflect the new workflow:

1. Automatic Setup: Explain that npm install downloads the pre-built Knowledge Base artifact.
2. Manual Download: Document npm run ai:download-kb for troubleshooting.
3. API Key: Clarify that the Gemini API Key is primarily for the Agent interaction and incremental KB updates, not strictly required for the initial KB existence anymore.
4. Rate Limits: Update the "Note on Cost" to mention the strict daily limits if users choose to rebuild the KB manually (npm run ai:sync-kb).

Acceptance Criteria:

• The guide accurately guides a new user through the "Zero-Setup" process.
• Misleading claims about speed and rate limits are corrected.