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/auth.rs

@@ -40,7 +40,7 @@ pub fn generate_token() -> String {
     base64::engine::general_purpose::URL_SAFE_NO_PAD.encode(bytes)
 }
 
-/// What `config.json` stores instead of the token: hex SHA-256. A plain
+/// What `config.ron` stores instead of the token: hex SHA-256. A plain
 /// hash is enough for high-entropy random input, and buys that a leaked
 /// config doesn't leak the credential.
 pub fn token_hash_hex(token: &str) -> String {
@@ -104,7 +104,7 @@ mod tests {
 
     fn manager_with_token(dir: &std::path::Path, token: &str) -> Arc<SessionManager> {
         let manager = Arc::new(
-            SessionManager::new(dir.join("config.json"), dir.join("sessions"))
+            SessionManager::new(dir.join("config.ron"), dir.join("sessions"))
                 .expect("manager"),
         );
         manager