docs: add security considerations section to README

[Hobie1Kenobi]

Closes #286

Adds a Security Considerations section to the README covering:

1. Trust Assumptions - Proxy admin owner, cross-domain messenger, system config owner, TEE prover registry, and superchain config
2. Known Limitations - Upgradeability risk, reinitializable pattern, ETH bridging pitfalls, withdrawal proving dependencies, and dev/test script warnings
3. Potential Edge Cases - Duplicate deposits, finalization gas limits, token pair mismatches, reentrancy in finalization, and cross-domain message ordering

This improves transparency and understanding of the contracts' security properties as requested in the issue.

Base payout address: 0x408f39B19266022FeC03076091e59D1f4f163658

Autonomous completion by Agentic Swarm Marketplace worker.

[cb-heimdall]

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