docs: document channel config and structured logging

Add per-channel plugin control and JSON logging sections to USAGE.md,
CHEATSHEET.md, and derp.toml.example. Mark items done in ROADMAP.md.

docs/CHEATSHEET.md

@@ -31,6 +31,25 @@ rate_limit = 2.0     # Messages per second
 rate_burst = 5       # Burst capacity
 ```
 
+## Per-Channel Plugin Control
+
+```toml
+# Only allow specific plugins in a channel
+[channels."#public"]
+plugins = ["core", "dns", "cidr", "encode"]
+
+# Omit section entirely to allow all plugins
+```
+
+`core` always active. PMs unrestricted. Denied commands silently ignored.
+
+## Structured Logging
+
+```toml
+[logging]
+format = "json"    # JSONL output (default: "text")
+```
+
 ## Container
 
 ```bash