feat(config): support custom cost tiers

[richardsun0713]

Issue for this PR

Closes #50713

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds cost.tiers to legacy provider model configuration and carries it through v1 migration and both provider paths. Configured tiers replace a matching context boundary while preserving other catalog tiers. Explicit tiers take precedence over the legacy context_over_200k shorthand at the same boundary.

How did you verify your code works?

• bun test test/config/config.test.ts test/config/provider.test.ts
• bun test test/provider/provider.test.ts
• bun typecheck in packages/core and packages/opencode
• Push hook: repository typecheck

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found the following related PRs (excluding PR #50717 itself):

Potential duplicates/related work:

1. PR refactor(opencode): normalize legacy pricing into tiers #44292 - refactor(opencode): normalize legacy pricing into tiers

   • Directly related: addresses similar tier normalization and legacy pricing concerns

2. PR fix(session): stop the legacy 200k price from overriding a model's real context tier #44223 - fix(session): stop the legacy 200k price from overriding a model's real context tier

   • Related to the legacy 200K shorthand preservation mentioned in this PR

3. PR feat(agent): implement model tier selection with variant support for subagents #11377 - feat(agent): implement model tier selection with variant support for subagents

   • Related to cost tier configuration and model selection

Why they're related:

• All address context tier configuration, legacy pricing handling, and cost model management
• PR refactor(opencode): normalize legacy pricing into tiers #44292 shares similar goals of normalizing pricing into tiers
• PR fix(session): stop the legacy 200k price from overriding a model's real context tier #44223 directly addresses the legacy 200K shorthand behavior you're preserving
• These PRs appear to be in the same feature area (provider cost configuration)

Consider reviewing PR #44292 and #44223 to ensure your changes don't conflict with or duplicate existing work on cost tier configuration.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[richardsun0713]

This is not a duplicate of #44223 or #44292. Those changes address runtime handling of catalog tiers; this PR adds the missing cost.tiers config surface for custom provider models and verifies its migration and merge behavior.