docs: add Coldcard Mk2/Mk3 seed-entropy security warning to README

[nrobi144]

Summary

Adds a security note under the Coldcard entry in apps/coordinator/README.md (which lists "Coldcard Mk2, Mk3, & Mk4").

In July 2026 Coinkite disclosed a firmware build error that reduced generated-seed entropy on Coldcard Mk3 (firmware 4.0.1–5.0.3) and pre-fix Mk4/Q. Since the README names the affected Mk2/Mk3 explicitly, this adds a one-line sourced warning next to it. Docs-only change.

Refs #523.

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Project	Deployment	Actions	Updated (UTC)
caravan-coordinator	Ready	Preview	Aug 7, 2026 5:18am

[Shadouts]

Thank you

[Shadouts]

I see what you changed. Admittedly, it's difficult to interpret what they are describing between their blogpost and their firmware download page which has two versions of mk4 firmware. But I think this description is fine.

[Shadouts]

run npm run changeset to add details about your change for version bumping purpose.