docs: update docs for multi-server support

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

docs/USAGE.md

@@ -23,7 +23,9 @@ derp --config /path/to/derp.toml --verbose
 
 ## Configuration
 
-All settings in `config/derp.toml`:
+All settings in `config/derp.toml`.
+
+### Single-Server (Legacy)
 
 ```toml
 [server]
@@ -67,6 +69,55 @@ port = 8080                   # Bind port
 secret = ""                   # HMAC-SHA256 shared secret (empty = no auth)
 ```
 
+### Multi-Server
+
+Connect to multiple IRC servers from a single config. Plugins are loaded
+once and shared; state is isolated per server (`data/state-<name>.db`).
+
+```toml
+[bot]
+prefix = "!"                  # Shared defaults for all servers
+plugins_dir = "plugins"
+admins = ["*!~root@*.ops.net"]
+
+[servers.libera]
+host = "irc.libera.chat"
+port = 6697
+tls = true
+nick = "derp"
+channels = ["#test", "#ops"]
+
+[servers.oftc]
+host = "irc.oftc.net"
+port = 6697
+tls = true
+nick = "derpbot"
+channels = ["#derp"]
+admins = ["*!~admin@oftc.host"]  # Override shared admins
+
+[logging]
+level = "info"
+format = "json"
+
+[webhook]
+enabled = true
+port = 8080
+secret = "shared-secret"
+```
+
+Each `[servers.<name>]` block may contain both server-level keys (host,
+port, tls, nick, etc.) and bot-level overrides (prefix, channels, admins,
+operators, trusted, rate_limit, rate_burst, paste_threshold). Unset keys
+inherit from the shared `[bot]` and `[server]` defaults.
+
+The server name (e.g. `libera`, `oftc`) is used for:
+- Log prefixes and `!version` output
+- State DB path (`data/state-libera.db`)
+- Plugin runtime state isolation
+
+Existing single-server configs (`[server]` section) continue to work
+unchanged. The server name is derived from the hostname automatically.
+
 ## Built-in Commands
 
 | Command | Description |
@@ -358,8 +409,8 @@ keys = bot.state.keys("myplugin")
 bot.state.clear("myplugin")
 ```
 
-Data is stored in `data/state.db` (SQLite). Each plugin gets its own
-namespace so keys never collide.
+Data is stored in `data/state-<name>.db` (SQLite, one per server). Each
+plugin gets its own namespace so keys never collide.
 
 ### Inspection Commands (admin)