A browser 'Subscribe' button is labelled Purchase even when it's free #378

Found while testing #367. _LABEL_RULES in sentient/browser/safety.py puts subscribe under Purchase. Clicking "Subscribe to newsletter" therefore shows "Make a purchase" with "Yes, buy it" / "Don't buy" buttons. That is wrong and alarming for a free sign-up.

Purchase also drives "Purchases always ask" (rules.py, loop.py), so the fix must keep a yes for anything that may cost money.

Proposal

• A free-looking subscribe (newsletter, updates, follow, notify me, with no price, "trial", "/month", "plan" or currency on or near the button) gets its own "Signs up" label. It still asks under the normal rules, but with honest wording.
• Anything with price or plan signals stays Purchase and always asks.
• Unit tests for both, plus a real check against a local page with a newsletter button and a "Subscribe – ₹499/month" button.