fix: v0.2.1 FIDO2 macOS keygen, grouped prompts, Linux hints

Use Homebrew ssh-keygen for FIDO2 key generation on macOS instead of
searching for libsk-libfido2.dylib (removed in modern openssh). Group
interactive apply prompts into 6 categories with explanations. Fix
Linux gitleaks install hint to show apt/dnf instead of brew.

Co-Authored-By: Claude <noreply@anthropic.com>

test/git-harden.bats

@@ -1148,7 +1148,7 @@ EOF
 # v0.2.0: Version bump
 # ===========================================================================
 
-@test "--version reports 0.2.0" {
+@test "--version reports 0.2.1" {
     run bash "$SCRIPT" --version
-    assert_output --partial "0.2.0"
+    assert_output --partial "0.2.1"
 }