Use Effect Schema to decode Codex structured outputs

- Replace manual JSON schema constants and parse helpers with `Schema.Struct` definitions
- Generate JSON Schema from Effect Schema and decode output via `Schema.fromJsonString`
- Map `SchemaError` to `TextGenerationError` for invalid Codex responses
- Update `ProviderCommandReactor` test mock typing for `generateBranchName`
e64ad4b2c7Julius Marminge committed on 3/1/2026, 7:43:32 AMparent0e146e2
2 files changedLine totals unavailable