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:

docs/PUBLIC_READINESS.md

@@ -101,6 +101,29 @@ dosh import-ssh palav homelab
 This appends entries to `~/.config/dosh/hosts.toml` without trying to become an
 OpenSSH config parser.
 
+## Optional Command Extensions
+
+Dosh can expose companion tools without taking a dependency on them. Command
+extensions live in client or host config and expand only the first trailing word:
+
+```toml
+[extensions.tm]
+command = "tm {args}"
+description = "Open the server-side tmux dashboard"
+```
+
+With that config, `dosh palav tm` runs `tm` in the remote Dosh shell and
+`dosh palav tm dosh` runs `tm 'dosh'`. Removing the table removes the integration.
+Host config can override a global extension, or disable it:
+
+```toml
+[palav.extensions.tm]
+command = "/opt/tm/bin/tm {args}"
+
+[other-host.extensions.tm]
+disabled = true
+```
+
 ## Forwarding (Implemented)
 
 SSH forwarding cannot be copied by keeping the bootstrap SSH connection open,