Add optional command extensions

docs/NATIVE_V1_SPEC.md

@@ -51,6 +51,7 @@ dosh -L 8080:127.0.0.1:80 host
 dosh -R 9000:127.0.0.1:9000 host
 dosh --session work host
 dosh --view-only --session work host
+dosh host tm
 ```
 
 Compatibility expectations:
@@ -107,6 +108,8 @@ Must work in v1:
 - `dosh update`.
 - `dosh doctor host` for config/auth/UDP reachability diagnostics.
 - `dosh sessions host` for session visibility.
+- Optional command extensions such as `dosh host tm` for companion tools that are
+  installed separately from Dosh.
 
 Should work in v1 if it does not compromise the transport schedule:
 
@@ -556,6 +559,10 @@ forward_agent = false
 send_env = ["LANG", "LC_*", "TERM", "COLORTERM"]
 set_env = {}
 forwardings = []
+
+[extensions.tm]
+command = "tm {args}"
+description = "Open an optional server-side tmux dashboard"
 ```
 
 Server: