feat: add OpenCode Zen Go official usage quota provider

[abduznik]

What

Adds an OpenCode Zen Go quota panel to the usage tab — the official cloud usage feed (GET https://opencode.ai/zen/go/v1/usage, same data as the web panel) showing rolling (5-hour), weekly and monthly utilisation percentages with their reset times, alongside the existing Claude/Codex limits cards.

This covers OpenCode usage that isn't visible in local logs — the OpenCode provider already tracks the local opencode.db, but token burns through Zen Go API providers (e.g. Hermes Agent pointed at opencode.ai/zen/go/v1) are only visible on the cloud-side quota feed.

How it works

• New providers::opencode_go module:
  • fetch_zen_go_usage(key) — one ureq GET, Authorization: Bearer, browser User-Agent (the endpoint 403s the default ureq UA), 5 s timeout, 60 s result cache (same cadence as claude_limits).
  • resolve_api_key() — OPENCODE_GO_API_KEY env var first, falling back to ${HERMES_HOME}/.env (the Hermes portable convention) so the quota lights up automatically under that setup without any extra config.
  • Pure parse_zen_go_usage(Value) parser with unit tests against the real response shape (usage.rolling/weekly/monthly with status/percent/resetsAt).
• app.rs — StateInner and UsageView carry ZenGoUsage; refresh() fetches it lock-free alongside the Claude limits; missing key → None → panel hidden.
• +page.svelte — ZenGoLimits types and a quota card mirroring the Claude limits card (labels, percent, track fill). Rendered only when at least one bucket is present.
• README + version bump 0.4.1 → 0.4.2.

Testing

• 351 tests pass, 0 fail.
• Includes a live-fetch test that runs against the real endpoint when a key is configured and self-skips otherwise (safe for CI): providers::opencode_go::tests::test_live_fetch_zen_go_usage — verified green against the live API.
• npm run check (svelte-check / tsc): 0 errors.

Notes

• Only /usage exists on the Zen Go API — no raw token counts or other endpoints, so the quota is percent + reset time, matching the web panel.
• Which is exactly why it's a limits-style card, not a tokens row.