fix: show error result in sanitized SDK output

[Dave-London]

Summary

• Includes the result field in sanitized SDK output when is_error is true
• Non-error results remain hidden to protect sensitive output
• Exports sanitizeSdkOutput for testability and adds 5 unit tests

Problem

When a Claude SDK execution fails (e.g., invalid model ID, API errors), the sanitizeSdkOutput function strips the result field from the output. This hides the actual error message, leaving users with only an opaque "process exited with code 1" and minified code context.

Users see:

{"type": "result", "is_error": true, "duration_ms": 230}

Instead of:

{"type": "result", "is_error": true, "result": "API Error: 404 model not found", "duration_ms": 230}

Changes

• base-action/src/run-claude-sdk.ts: Conditionally spread result when is_error is true, with "result" in resultMsg type guard for the SDKResultSuccess/SDKResultError union
• test/sanitize-sdk-output.test.ts: 5 tests covering error result inclusion, success result omission, init messages, and message suppression

Test plan

• 5/5 sanitize-sdk-output tests pass
• TypeScript typecheck passes
• Prettier formatting passes

Fixes #880