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

app/androidApp/src/main/kotlin/com/example/aiapp/Api.kt

@@ -130,7 +130,7 @@ fun fetchSessions(settings: ServerSettings): List<SessionSummary> =
     requestFromServer(settings, "/sessions") { it.jsonObjects(::parseSession) }
 
 // What the server offers, so the spawn screen has no hardcoded lists: a
-// provider or host added to the server's config.json appears here with no
+// provider or host added to the server's config.ron appears here with no
 // app rebuild.
 data class Provider(val name: String, val kind: String, val models: List<String>)
 

app/androidApp/src/main/kotlin/com/example/aiapp/SpawnScreen.kt

@@ -45,7 +45,7 @@ private const val LOCAL_HOST_LABEL = "backend"
  * The spawn screen: what to run, where to run it, and the per-kind fields.
  *
  * Providers and hosts both come from the server, so adding either to its
- * config.json shows up here with no app rebuild -- and because they are
+ * config.ron shows up here with no app rebuild -- and because they are
  * independent, any provider can be sent to any host.
  */
 @Composable