docs: add GitHub Dependabot comparison to comparison.md

[sonukapoor]

Adds a dedicated CVE Lite CLI vs GitHub Dependabot section to docs/comparison.md, prompted by issue #249 where a user compared results and defaulted to trusting Dependabot.

• Adds Dependabot as a column in both comparison tables
• Explains why scan results may differ (OSV ingests GHSA, so it's not a coverage gap — the real causes are ingestion timing, version range matching, and transitive classification methodology)
• Documents five concrete areas where CVE Lite goes further: developer-time scanning, validated fix commands, fix version validation, usage-aware reachability, and offline support
• Honest about where Dependabot has the edge (automation, multi-ecosystem, GitHub-native UI)
• Closes with a complementary use framing

Closes #254