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.

config/derp.toml.example

@@ -23,3 +23,15 @@ plugins_dir = "plugins"
 
 [logging]
 level = "info"
+# format = "json"         # Log format: "text" (default) or "json" (JSONL)
+
+# -- Per-Channel Plugin Control --
+# Channels without a section run all plugins.
+# Channels with a `plugins` list only run those plugins.
+# The `core` plugin is always active regardless of config.
+
+# [channels."#public"]
+# plugins = ["core", "dns", "cidr", "encode"]
+
+# [channels."#ops"]
+# plugins = ["core", "revshell", "payload", "exploitdb", "opslog"]