Closes #18 Implemented: add security threat model for secret handling and transaction flows #18

[Promise278]

Closes #18

Summary

Adds a comprehensive security threat model (docs/SECURITY_THREAT_MODEL.md) to AnchorKit covering all major threat areas identified across the monorepo.

What's covered

• 9 threat areas: Secret key handling, payment intent validation, transaction readiness, anchor metadata/lifecycle, account diagnostics, Soroban escrow contract, unsafe logging, mainnet risks, web dashboard
• 47 threats catalogued with severity, current mitigations, and open gaps
• Consolidated risk register — 6 critical/high, 8 medium, 10 low
• Out-of-scope production behaviours — explicit list of what MVP does NOT do (no custody, no signing, no secure storage, etc.)
• Review requirements — pre-merge and pre-mainnet checklists

Changes

• docs/SECURITY_THREAT_MODEL.md — new file (472 lines)
• README.md — added threat model to docs index table

Why

Security assumptions were spread across README notes, code comments, and contributor expectations. This document consolidates them into a single source of truth with clear threat actors, attack surfaces, mitigations, and gaps.