feat: add skill fixture test harness

[kamalsrini]

Summary

• Adds a deterministic static fixture test harness
• Validates fixture manifests, target paths, expected findings, and evidence strings
• Adds CI workflow for fixture tests
• Documents the fixture contract and local test command
• Includes a tiny _example fixture so the harness is executable immediately

Design

This intentionally does not invoke a live agent/model in CI. It validates static fixture contracts now; live execution adapters can be added later.

Linear

• UNI-9

Verification

• ruby scripts/test_skill_fixtures.rb
• ruby -c scripts/test_skill_fixtures.rb
• ruby scripts/validate_skill_schema.rb
• ruby scripts/validate_index.rb
• git diff --check HEAD