feat(cli): add config migrations, releases, and report comparisons

[eliasaph01]

Summary

Implements backward-compatible CLI configuration migrations, signed multi-platform CLI releases, and historical scan report comparisons.

Changes

• Migrate legacy config keys in memory and reject invalid or unsupported schema versions.
• Build standalone CLI executables for Linux x64, macOS x64/arm64, and Windows x64, with Sigstore signature bundles.
• Add gasguard compare <baseline.json> <current.json> with text/JSON output and optional file output.
• Repair existing CLI build and scan-discovery blockers required for standalone releases.
• Document migration behavior, report comparison, release dependencies, and signature verification.

Verification

• 24 CLI tests passed.
• CLI TypeScript build check passed.
• Release workflow YAML validated with four platform targets.
• Built and ran a standalone macOS x64 executable with gasguard --help.

Closes #1112
Closes #1144
Closes #1145
Closes #1148

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