Add unit tests for ai/text.js truncation helper

[false200]

What is this about?

Problem

trimTextForApi() caps OCR text at 14,000 chars before sending to AI providers. No tests verify this behavior.

Where to look

• ai/text.js — trimTextForApi, MAX_API_TEXT_CHARS
• New file: scripts/test-text.mjs
• Wire into npm test (see related markdown test issue)

Acceptance criteria

• Short text returned unchanged
• Text over 14,000 chars truncated with [truncated to save tokens]
• Edge cases: empty string, exactly 14,000 chars

Skills needed

Basic Node assertions — pair nicely with the markdown test issue.