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>

README.md

@@ -82,6 +82,7 @@ All connection and collection settings live in a single JSON file:
 | `host` | `"0.0.0.0"` | Push server bind address |
 | `port` | `9999` | Push server TCP port |
 | `log_dir` | `"./logs"` | Directory for rotating JSON log files |
+| `diff` | `false` | Enable diff mode for push server transmission |
 
 ### CLI reference
 
@@ -97,7 +98,7 @@ CLI flags override the corresponding credentials file value when given.
 | `--port PORT` | `port` | Push server TCP port |
 | `--log-dir DIR` | `log_dir` | Directory for rotating log files |
 | `--max-records N` | — | Keep only the last N records per file |
-| `--diff` | — | Send diffs instead of full snapshots (must match client setting) |
+| `--diff` | `diff` | Send diffs instead of full snapshots (must match client setting) |
 | `--dry` | — | Skip WeConnect; run push server only (for testing) |
 | `-v` | — | Verbose / debug logging |
 | `--list-domains` | — | Print available domains and exit |