Add Display trait for SuspicionReason enum

[jason-h23]

Summary

The SuspicionReason enum in src/sentinel/types.rs has a score() method but no human-readable Display implementation. Adding one improves debugging, CLI output, and alert readability.

Current State

Each variant contains diagnostic data (addresses, gas values, counts) but Debug output is the only way to print them.

What To Do

Implement Display for SuspicionReason so each variant produces a readable string like:

• "Flash loan detected from 0x7d27..., score 0.4"
• "High value revert: 50.5 ETH, 250000 gas used"

Files

• src/sentinel/types.rs (lines 48-64)

Acceptance Criteria

• Display trait implemented for all variants
• Output is human-readable (not Debug format)
• Existing tests still pass (cargo test --all-features)
• cargo clippy --all-features -- -D warnings passes

Estimated Effort

~30-50 lines, 1-2 hours