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/session/claude.rs

@@ -37,7 +37,7 @@ use crate::config::{HostConfig, ProviderConfig, SessionConfig};
 /// Where the driver remembers its CLI session id between backend runs --
 /// the whole crash-recovery story: respawning with `--resume <id>` picks
 /// the conversation back up from Claude's own session files. Kept in the
-/// session directory rather than config.json so the shared schema stays
+/// session directory rather than config.ron so the shared schema stays
 /// free of per-driver state.
 const RESUME_FILE: &str = "claude-session.json";
 

server/src/session/mod.rs

@@ -1,6 +1,6 @@
 //! The live session registry. Every session mutation -- spawn, delete,
 //! token changes -- funnels through [`SessionManager`] under one lock, so
-//! in-memory state and `config.json` can't come apart (the same pattern as
+//! in-memory state and `config.ron` can't come apart (the same pattern as
 //! dev-updater's `registry.rs`).
 //!
 //! A live session is a driver plus one event pump: the driver reports
@@ -566,7 +566,7 @@ mod tests {
     #[tokio::test]
     async fn spawn_message_and_delete_round_trip() {
         let dir = tempfile::tempdir().expect("tempdir");
-        let config_path = dir.path().join("config.json");
+        let config_path = dir.path().join("config.ron");
         let data_dir = dir.path().join("sessions");
         let manager = SessionManager::new(config_path.clone(), data_dir.clone()).expect("manager");
 
@@ -619,7 +619,7 @@ mod tests {
     async fn questions_round_trip_through_answer() {
         let dir = tempfile::tempdir().expect("tempdir");
         let manager = SessionManager::new(
-            dir.path().join("config.json"),
+            dir.path().join("config.ron"),
             dir.path().join("sessions"),
         )
         .expect("manager");
@@ -651,7 +651,7 @@ mod tests {
     #[tokio::test]
     async fn a_restart_relaunches_sessions_and_continues_the_numbering() {
         let dir = tempfile::tempdir().expect("tempdir");
-        let config_path = dir.path().join("config.json");
+        let config_path = dir.path().join("config.ron");
         let data_dir = dir.path().join("sessions");
 
         let manager = SessionManager::new(config_path.clone(), data_dir.clone()).expect("manager");