feat: add scan findings diff between runs (--compare-to)

[kai-agent-free]

Closes #88

Summary

Adds --compare-to <baseline.json> flag to g0 scan that shows new, fixed, and unchanged findings compared to a previous scan.

Implementation

• src/compare.ts — Core diff logic: fingerprinting (ruleId + file + snippet hash), baseline loading, finding comparison
• src/reporters/compare-terminal.ts — Color-coded terminal output showing new ⚠, fixed ✓, and unchanged findings
• src/cli/commands/scan.ts — --compare-to option wired into scan command

Usage

# Save baseline
g0 scan . --json > baseline.json

# Compare later
g0 scan . --compare-to baseline.json

# JSON diff output
g0 scan . --compare-to baseline.json --json

Features

• Finding fingerprinting via SHA-256 of ruleId + file + snippet (stable across line shifts)
• Terminal diff: new findings (red), fixed findings (green), unchanged count
• Score delta display when baseline has score data
• JSON diff output with --json flag
• Exit code 1 when new critical/high findings found (CI-friendly)
• 10 unit tests covering fingerprinting, comparison, and baseline loading