Polish README presentation and trust signals

[Nathanielguo]

What changed

• Added concise product positioning and AI/ATS workflow boundaries.
• Clarified the distinction between local application state and external AI requests.
• Preserved every existing heading, core statement, link, and image.

Why it matters

Visitors can evaluate the workflow and data path more accurately, improving professional credibility without rewriting the project's existing content.

Verification

• Only README.md changed
• Additive-only diff; 0 deleted lines
• Markdown preservation validator passed

Draft review only. This PR will not be merged automatically.