collect: add diff to credentials file; --diff overrides it

Change --diff from store_true to default=None so CLI presence is
distinguishable from the JSON default. If not given on CLI, reads
creds_data.get("diff", False). Updated install.sh template and README.

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

collect.py

@@ -220,7 +220,8 @@ def main() -> None:
     p.add_argument(
         "--diff",
         action="store_true",
-        help="Send diffs instead of full snapshots over the push server (default: off)",
+        default=None,
+        help="Send diffs instead of full snapshots over the push server (overrides credentials file, default: off)",
     )
     p.add_argument(
         "--dry",
@@ -273,6 +274,8 @@ def main() -> None:
         args.port = int(creds_data.get("port", 9999))
     if args.log_dir is None:
         args.log_dir = creds_data.get("log_dir", "./logs")
+    if args.diff is None:
+        args.diff = bool(creds_data.get("diff", False))
 
     if not args.dry and (not args.username or not args.password):
         p.error(