Remove in-repository CLI

[jakthom]

Summary

• remove the shipped cmd/n2k command, its tests, demo, and build recipe
• drop Cobra and GoReleaser dependencies and publish source-only GitHub releases
• update library documentation, architecture ownership, changelog, and stale binary ignore rules
• direct command-line users to the separate open-ships/n2k-cli repository

Maintainer-only cmd/pgngen and cmd/roundtrip utilities remain in place.

Validation

• go test ./...
• just pgn-sync-check
• just lint
• just secure