docs: old product name F∆I Platform -> Ch∆In + contact chain@flemming.ai

The old 'F∆I Platform' product name and platform@flemming.ai contact
survived in docs/NOTICE/descriptions/help text; the product is Ch∆In and
the contact is chain@flemming.ai. Generic 'cross-platform/platform-native'
left untouched.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

.githooks/commit-msg

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# F∆I Platform — commit-msg hook.
+# Ch∆In — commit-msg hook.
 # Validates the commit message: Conventional Commits subject, DCO
 # sign-off, no Claude co-author trailer, no banned phrases.
 exec "$(git rev-parse --show-toplevel)/tools/security/check-staged.sh" message "$1"