[Libretto] implement skip/xfail annotation fields at file and case levels of the test model

[typotter]

Implement skip/xfail annotation fields at file-level and case-level in the test data schema. Skip prevents test execution; xfail marks tests as expected to fail. Both fields support SDK-scoped values to handle SDK-specific behavioral divergences and unimplemented features.

Acceptance Criteria

• All evaluation-cases/test-*.json files migrated from bare array to wrapper object format with cases key
• File-level skip and xfail fields supported on wrapper object
• Case-level skip and xfail fields supported on each case object
• SkipValue type: true | ["sdk", ...] | null
• XfailValue type: true | "reason string" | {"sdk": "reason", ...} | {"sdk": true, ...} | null
• Canonical SDK ids: "go", "java", "python", "dotnet"
• Case-level annotations override file-level; null at case-level opts back in to normal run
• Skip takes precedence over xfail when both apply
• README.md updated to document new schema
• CLAUDE.md updated to document new schema
• At least one example file demonstrates annotations at both file and case levels

[leoromanovsky]

Cool!