Add optional command extensions

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,