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/routes.rs

@@ -111,7 +111,7 @@ async fn list_sessions(State(manager): State<Arc<SessionManager>>) -> axum::Json
 }
 
 /// What the spawn screen needs to render itself, so the phone holds no
-/// hardcoded list: an entry added to `config.json` shows up with no app
+/// hardcoded list: an entry added to `config.ron` shows up with no app
 /// rebuild. Providers and hosts are listed separately because they are
 /// independent choices -- any provider can be run on any host.
 #[derive(serde::Serialize)]