Add a rich social source skill fixture

[kriptoburak]

Summary

• add a richer valid skill fixture for source-backed social approval workflows
• cover references, examples, metadata, license, validation, troubleshooting, and gotchas in the fixture
• assert the fixture keeps Discoverability at 100 and an A-grade composite score

Validation

• changed-file rustfmt check
• cargo test all
• cargo clippy all-targets with warnings denied
• skillmark JSON check for the new fixture

Note: full workspace cargo fmt check still reports pre-existing formatting drift outside this change.