fix: estimate cost when stored zero is placeholder

[alanzchen]

Why: some OpenCode setups store zero cost for nonzero-token responses, which makes oc-stats show zero cost everywhere because it always trusts stored cost values. Fix: estimate pricing when stored cost is zero on nonzero-token events by default, add a --keep-zero flag to preserve the original behavior, and cover the new behavior with tests and docs.

[Copilot]

Pull request overview

This PR fixes a cost-reporting edge case where OpenCode stores cost: 0 as a placeholder despite nonzero token usage, causing oc-stats to display $0 everywhere by always trusting stored costs. It introduces a default behavior to estimate costs when a stored zero looks like a placeholder, with an opt-out CLI flag to preserve the previous “trust stored zero” behavior.

Changes:

• Add ZeroCostBehavior and centralized update_price_summary helper to resolve stored-vs-estimated cost consistently.
• Thread zero_cost_behavior through analytics snapshot + UI app wiring, and add --keep-zero CLI flag.
• Add/adjust tests and document the new default + opt-out behavior in the README.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/utils/pricing.rs	Introduces ZeroCostBehavior and update_price_summary with unit tests for placeholder-zero handling.
src/ui/app.rs	Stores and propagates zero_cost_behavior into snapshot recomputation.
src/main.rs	Adds --keep-zero flag and maps it to ZeroCostBehavior passed into the app.
src/analytics/model_stats.rs	Switches model/provider cost accumulation to the shared update_price_summary helper.
src/analytics/mod.rs	Updates snapshot building to use update_price_summary and threads zero_cost_behavior through chart builders; updates/adds tests.
README.md	Documents the new default estimation behavior and the --keep-zero opt-out.

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[Cateds]

There's a clear counterexample: model calls under OpenAI Codex subscription. They would have issues with this strategy.

When using OAuth to log into Codex, all cost values for openai provider model calls (like gpt-5.4) are correctly recorded as 0. Treating 0 as a placeholder would fallback to models.dev's OpenAI API-based pricing logic. The result would be a discrepancy exactly matching my Codex usage cost compared to what opencode stats and opencode stats --models showed when I was debugging. The current logic is at least consistent with OpenCode's own statistics in my case.

I'm curious what you mean specifically by "some OpenCode setups store zero cost for nonzero-token responses". The previous default behavior should display something like $5.09 + ? for unknown providers (though after fully trusting 0, that fallback code path did become dead code). Do you have a more specific example? I haven't verified whether configuring cost in opencode.json would also write to the database's cost field.

[alanzchen]

My motivation was using this with my OpenAI Codex subscription. I wanted to estimate how much it would have cost me if I didn't have the subscription. That being said, I agree that a more consistent design would be the current behavior by default but add an --ignore-zeros option.

[Cateds]

I'm sorry for the delay. I changed it to default Keep Zero, and then optionally ignore Zero.