Add ccc-keygen for secrets file generation

--create <names>  generates secrets.json with random 32-byte hex secrets
--filter <names>  extracts a subset of users into filtered-secrets.json
--input/--output  override default file paths

Removes hardcoded user/agent assumptions from secrets.example.json.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -10,7 +10,8 @@
   "bin": {
     "ccc-server": "bin/ccc-server.mjs",
     "ccc-client": "bin/ccc-client.mjs",
-    "ccc-queue":  "bin/ccc-queue.mjs"
+    "ccc-queue":  "bin/ccc-queue.mjs",
+    "ccc-keygen": "bin/ccc-keygen.mjs"
   },
   "dependencies": {
     "blessed": "^0.1.81",