feat(vulnera): drop Snyk support (#325)

[HoyeongJeon]

Drops all Snyk-related code as Snyk APIs are locked behind a paywall and maintaining support is no longer viable.

Removed components:

• Strategy (src/strategies/snyk.ts) — Snyk vulnerability fetching strategy
• Database (src/database/snyk.ts) — Snyk API client class
• Format (src/formats/snyk/) — Snyk-specific vulnerability format and mappers
• VULN_MODE.SNYK constant removed from src/constants.ts
• Residual mapFromSnyk code removed from standard format mappers
• All related tests, fixtures, and documentation
• Updated outdated origin comment in docs/formats/standard.md (Snyk, NodeSWG → Sonatype, GitHub, OSV)

Closes #325

[fraxken]

That's not caused by your PR, but can you add a commit to fix lint issues? Thanks

[PierreDemailly]

@HoyeongJeon tests fails due to last commit

[fraxken]

Can you please squash the two last commits together ?

[fraxken]

@allcontributors please add @HoyeongJeon for code

[allcontributors]

@fraxken

I've put up a pull request to add @HoyeongJeon! 🎉