docs(security): add sample supply chain analysis for webapp01

Sample findings produced by the custom SupplyChainSecurityAgent subagent demonstrating supply chain security analysis capabilities on src/webapp01.

Reports added under security-reports/:

- supply-chain-report.md (full analysis)

- pr-ready-fixes.md (diff-ready remediations)

- engineering-backlog.md (18 sprint-ready work items)

- quick-reference.md (executive summary)

Closes #55

Author: emmanuelknafo