Fix custom-agent eval example schema

[spboyer]

This updates the checked-in custom-agent example so it matches the current eval and task schemas and can be copy/paste run with the current CLI.

• Adds the required eval top-level fields and switches the example to a supported executor
• Updates the prompt grader to the current config shape
• Rewrites the example task files to the current task schema and references the fixture files directly
• Adds a regression test to keep the checked-in example schema-valid

Validation: go test ./...

Docs impact: none beyond the example and schema regression coverage.

Closes #274

[Copilot]

Pull request overview

Updates the checked-in examples/custom-agent eval suite to match the current eval/task JSON schemas, and adds a regression test to ensure the example remains schema-valid as the schemas evolve.

Changes:

• Add a schema regression test that validates examples/custom-agent/eval.yaml and its referenced task files.
• Update the custom-agent example eval.yaml to include required top-level fields (skill, version, metrics) and the current prompt-grader config shape.
• Rewrite the example task YAMLs to the current task schema, including attaching fixture files via inputs.files.

Show a summary per file

File	Description
internal/validation/schema_test.go	Adds a regression test to validate the checked-in custom-agent example against the current schemas.
examples/custom-agent/eval.yaml	Updates eval spec to current schema (required fields + grader config shape).
examples/custom-agent/tasks/review-clean-code.yaml	Migrates task to current task schema and attaches fixture file input.
examples/custom-agent/tasks/find-xss.yaml	Migrates task to current task schema and attaches fixture file input.
examples/custom-agent/tasks/find-sql-injection.yaml	Migrates task to current task schema and attaches fixture file input.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2