feat(usage): enforce one free account per install (fair-use policy)

[Fahad090NP]

📝 What does this change?

A single-free-account fair-use policy so one person can't harvest OpenCode's free quota by registering multiple free accounts (inference is expensive for everyone).

The rule: exactly one free account is allowed per install. The moment 2+ free accounts are detected, free-model usage is blocked with a clear, actionable error. Paid usage is never blocked — multiple paid accounts are fine. Delete down to 1 free account (or make the extra one paid) and free models work again.

How an account is classified (per API-key fingerprint):

• Free = a profile that uses a free model (*-free / big-pickle).
• Paid = has an active Go subscription (checked via the official /zen/go/v1/usage endpoint) or successfully uses a paid model → exempt from the free limit.
• Subscription lapsed? → Go accounts are re-classified within ~1 min via the usage sync; Zen accounts within 24h of the last paid use → they count as free again and the limit applies. Renewing re-confirms paid.

Coverage: new profiles and existing profiles (re-evaluated on every request), the Agents window, and vision-proxy requests. The block fires before the request is sent, so no quota is spent.

Code structure (4 commits, pure-logic separation):

1. Pure policy module src/usage/freeAccountPolicy.ts + storage wrapper + 11 unit tests (src/test/freeAccountPolicy.test.ts) covering: single free allowed, 2nd free blocks, paid exemption, lapsed-sub re-classification, TTL renewal, delete-account recovery, round-trip persistence.
2. Provider enforcement in provideLanguageModelChatResponse (+ paid confirmation on successful paid-model usage).
3. Go subscription status wired from the usage sync into the policy.
4. Delete Profile cleans the policy; the Usage quick-pick warns when free usage is blocked; Diagnostics now shows a "Free-Account Policy" section (free/paid fingerprints + blocked state) so the policy is transparent.

🧪 How did you test it?

• npm run lint — all 7 steps green, incl. the 11 new policy unit tests.
• Full gate re-run after each of the 4 commits.
• Logic verified by scenario tests (can't test live with 2 free accounts myself — the Diagnostics section lets anyone confirm the state).

✅ Checklist

• npm run compile passes
• npm run lint passes
• npm test passes
• npm run package produces a VSIX (not run — code + tests only)
• I tested it works (live multi-account verification still recommended)
• I updated docs/CHANGELOG if needed (left for release-time per repo convention)

⚠️ Honest limitations (please read)

1. Per-install deterrent, not identity proof. Two keys can't be proven to belong to one person; a determined user could bypass it on another machine or via the OpenCode CLI. It stops casual multi-account harvesting.
2. An account that has never used anything yet can't be pre-classified — it's counted the moment it first uses a free model.
3. Zen paid accounts have up to 24h grace after their last paid use (there's no subscription-check endpoint for Zen; Go has one, hence ~1 min).

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✨ PEACE BE UPON YOU 🤲

[Fahad090NP]

If I ever had a wish for any of my PR to be approved, then this is the one. Not because I am jealous of the people who are using this trick to scam OpenCode and I am not, but because both Islam and then the ethics forbids such acts strictly. It was always be a difficult thing to let others think what I actually want to say.

Fi Amanillah.

[Fahad090NP]

I do not want to hear anything other than any bug or inconsistency in my code. Either Merge, tell me what to improve, or Reject it.