fix: add FreeUsageLimitError to RETRYABLE_ERROR_NAMES set

[code-yeongyu]

Follow-up to #2422. Adds FreeUsageLimitError to structured error name set as identified by cubic code reviewer — aligns with OpenCode SDK error classification model.

---

Summary by cubic

Treats FreeUsageLimitError as retryable and makes error name checks case-insensitive, resolving Linear issue 2393 (Cubic error name not retried).

• Bug Fixes
  • Normalize error names to lowercase for both retryable and non-retryable checks, add "freeusagelimiterror" to the retryable set, and add tests for PascalCase/lowercase variants.

^{Written for commit 755efe2. Summary will update on new commits.}

[cubic-dev-ai]

3 issues found across 3 files

Confidence score: 2/5

• High risk: src/shared/model-error-classifier.ts lowercases error names while lookup sets use PascalCase, which will break classification for most errors.
• src/tools/background-task/create-background-task.ts awaits metadata() even though the SDK defines it as void, which could cause unexpected runtime behavior.
• Pay close attention to src/shared/model-error-classifier.ts and src/tools/background-task/create-background-task.ts - error classification logic and improper async usage.

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="src/tools/background-task/create-background-task.ts">

<violation number="1" location="src/tools/background-task/create-background-task.ts:101">
P1: Custom agent: **Opencode Compatibility**

The `metadata` method in the OpenCode SDK is typed as returning `void` and should not be awaited. It is designed to be a fire-and-forget update.</violation>
</file>

<file name="src/shared/model-error-classifier.ts">

<violation number="1" location="src/shared/model-error-classifier.ts:101">
P0: Converting the error name to lowercase breaks lookups because the sets contain PascalCase strings. This will break error classification for all other errors.</violation>
</file>

<file name="src/shared/model-error-classifier.test.ts">

<violation number="1" location="src/shared/model-error-classifier.test.ts:86">
P3: The test description incorrectly labels `FreeUsageLimitError` as lowercase.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[code-yeongyu]

[sisyphus-bot] Addressed the findings identified by cubic. Summary: (1) made error-name set matching consistent with case-insensitive lookup, (2) kept the contaminated await change removed, (3) fixed the misleading PascalCase test label. @cubic-dev-ai please re-review.

[cubic-dev-ai]

[sisyphus-bot] Addressed the findings identified by cubic. Summary: (1) made error-name set matching consistent with case-insensitive lookup, (2) kept the contaminated await change removed, (3) fixed the misleading PascalCase test label. @cubic-dev-ai please re-review.

@code-yeongyu I have started the AI code review. It will take a few minutes to complete.

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Correctly implements case-insensitive error matching and adds FreeUsageLimitError to the retryable set with corresponding tests, ensuring no regressions.}