Removes the legacy_targets YAML field and its install-time auto-migration
of pre-v6.1.0 directories (.claude/commands, .opencode/agents, etc.). On
install, surface a warning instead: read manifest version and scan 24
known legacy paths, then print rm -rf commands the user can run themselves.
Also deletes orphan tools/platform-codes.yaml (never loaded by any code)
and fixes a stale URL in the cs translation.
Brian Madison