[README] Tighten first-screen positioning and quick start

[Longfellow1]

Background

KeyPulse already has a strong product story, but first-time visitors need to understand the project in about 10 seconds: what it is, what it is not, and why it is worth saving.

Task

Review the README first screen and tighten the path from positioning to quick start.

Acceptance criteria

• First screen explains KeyPulse as a local-first Mac companion for Obsidian-native daily reflection
• First screen clearly says it is not a productivity dashboard, screen-time tracker, or cloud surveillance assistant
• Quick start is visible and matches the current install path
• Chinese product narrative is preserved below the English-first summary