Add 3 new graph reasoner templates

[cafzal]

Summary

Adapted from the supply chain hero journey questions crafted by Lizi:

• BOM Reachability — Trace transitive dependencies through a bill of materials using reachable(full=True) and identify structural bottlenecks with betweenness centrality
• Site Centrality Network — Assess supply chain resilience using weakly connected components, bridge detection, and eigenvector centrality on a weighted site dependency graph
• Supplier Impact Analysis — Map upstream supplier dependencies for high-value customers and trace downstream blast radius of a supplier disruption using parameterized reachability

Test plan

• BOM Reachability: pip install . && python bom_reachability.py — reachability results and betweenness centrality output verified
• Site Centrality Network: pip install . && python site_centrality.py — WCC, bridge detection, and eigenvector centrality output verified
• Supplier Impact Analysis: pip install . && python supplier_impact.py — upstream and downstream reachability plus betweenness output verified
• CSV data files load without errors for all three templates
• READMEs render correctly (frontmatter, code blocks, collapsible troubleshooting sections)

🤖 Generated with Claude Code

[somacdivad]

Thanks @cafzal for helping to get us templates for other kinds of reasoning! Just a couple of tiny fixes needed here for dependencies, then we'll be all good to merge.

[github-actions]

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