[feature] opencode-zen: remove billing page scraping once Zen usage API exists

[vmvarela]

Context

The opencode-zen provider currently scrapes the Zen workspace billing page (https://opencode.ai/workspace/[id]/billing) with a session cookie, regex-parsing SolidJS SSR HTML to extract balance, monthlyLimit, monthlyUsage, and lastPayment.

This is fragile — it breaks on every frontend change (3 parser fixes in recent weeks: #206, #211, #213). The billing page is not an API contract.

Blocked on

Upstream has no workspace billing API endpoint yet:

• GET /zen/go/v1/usage (feat(console): add go usage endpoint anomalyco/opencode#16513) covers Go plan usage windows only — plan-window percentages, no monetary amounts, no balance.
• Zen workspace balance API requested in Feature Request: Add Zen balance API endpoint anomalyco/opencode#10448, with an extension comment proposing the full shape we need (balance anchored to last top-up + monthly window, server-derived status): Feature Request: Add Zen balance API endpoint anomalyco/opencode#10448 (comment)

When the API lands

• Migrate opencode-zen to the usage API (Bearer API key, no cookie)
• Drop billing page scraping + all SSR/data-slot parsers from src/lib/opencode-zen.ts
• Remove the 100_000_000 units = $1 conversion constant (API returns decimal USD)
• Update tests and fixtures

[slkiser]

Maybe if I say that they won't fix it...