Fix: clarify README editing guidance

[ishaan-jaff]

Summary

The README Notes section used overly broad guidance that implied App.svelte is the only file contributors ever need to edit. This updates the note to describe App.svelte as the starting point for most UI changes and adds a small README phrasing check to prevent the unclear phrase from returning.

Repro

On main, run:

python3 - <<'PY'
from pathlib import Path
text = Path('README.md').read_text()
phrase = 'is all you need to edit'
print('README clarity check')
print(f'Forbidden unclear phrase: {phrase!r}')
if phrase in text:
    print('FAIL: README uses overly broad guidance.')
    raise SystemExit(1)
print('PASS: README avoids overly broad guidance.')
PY

This fails because the README contains is all you need to edit.

Evidence

Tests

• npm run check:readme - passed.
• npm run build - passed, with existing Svelte warnings about an unused CSS selector, an unassociated form label, and autofocus.
• npm run check - failed with 9 errors and 5 warnings in existing Svelte/TypeScript diagnostics in App.svelte, Providers.svelte, RequestForm.svelte, ProviderDropdown.svelte, and Main.svelte; these match the workspace's documented pre-existing svelte-check failures and are unrelated to this README-only change.