Follow dev-updater's own config to RON

The same move, for the same reason: this file is written and read by hand,
and JSON has no comments to say why a host is configured the way it is.
Both house rules come across with it, in config.rs's `format` module and
nowhere else -- a file is the *body* of the config, so no outer parentheses
and nothing indented for them, and `Some` is implicit, which is what makes
`skip_serializing_if` on every optional field load-bearing rather than
tidiness.

The switch is outright: there is no reader for the old format. That is
invisible everywhere except here, because this file holds the enrolled
token hashes -- starting empty leaves the phone unable to talk to the
server and looks, from the phone, like the config having been lost. So a
config.json left beside the new file is named in the log and left alone,
rather than read or deleted.

One wart, documented at DriverKind: the kebab-case spelling is the string
the phone compares against, so it stays, and the file pays for it with
`kind: r#claude-cli` -- a hyphen is not a RON identifier. Renaming the
variant would change what an already-installed build is talking to.

Verified: cargo test, cargo clippy --all-targets, and a real start against
a scratch state directory -- a hand-typed config with comments and a bare
`port: 2222` loads, and what the server writes back sits at column 0 with
no Some(...) in it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017xn8nHw1tw1R6PtiY1eEtw

server/src/main.rs

@@ -36,7 +36,7 @@ use session::SessionManager;
 const DEFAULT_PORT: u16 = 8443;
 const WG_INTERFACE: &str = "wg0";
 
-/// `$XDG_CONFIG_HOME/ai-app`, or `~/.config/ai-app`. Holds `config.json`
+/// `$XDG_CONFIG_HOME/ai-app`, or `~/.config/ai-app`. Holds `config.ron`
 /// and `certs/`.
 fn config_home() -> PathBuf {
     xdg_dir(std::env::var_os("XDG_CONFIG_HOME"), ".config")
@@ -77,7 +77,7 @@ struct Args {
     bind: Option<IpAddr>,
 
     /// Where the token hashes, providers, hosts, and session list live.
-    /// Defaults to `$XDG_CONFIG_HOME/ai-app/config.json`.
+    /// Defaults to `$XDG_CONFIG_HOME/ai-app/config.ron`.
     #[arg(long)]
     config: Option<PathBuf>,
 
@@ -171,7 +171,7 @@ async fn main() -> Result<()> {
     tracing_subscriber::fmt().with_env_filter("info").init();
     let args = Args::parse();
 
-    let config_path = args.config.unwrap_or_else(|| config_home().join("config.json"));
+    let config_path = args.config.unwrap_or_else(|| config_home().join("config.ron"));
     let data_dir = args.data_dir.unwrap_or_else(|| data_home().join("sessions"));
     let manager = Arc::new(
         SessionManager::new(config_path.clone(), data_dir)