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

AGENTS.md

@@ -33,13 +33,19 @@ repo is in PLAN.md's "Backend layout" section.
 - `server/` — Rust backend (`ai-server`). `main.rs` bootstraps (TLS, the
   auth layer, token/QR enrollment, wg0 binding), `routes.rs` has the HTTP
   table in its module doc comment, `auth.rs` the bearer-token middleware,
-  `config.rs` the persisted schema, `session/` the manager (registry
-  pattern), `Driver` trait + event model, `EchoDriver`, and transcripts.
+  `config.rs` the persisted schema and the RON its file is written in,
+  `session/` the manager (registry pattern), `Driver` trait + event model,
+  `EchoDriver`, and transcripts.
 - `app/` — Compose Android app, single `:androidApp` module, package
   `com.example.aiapp`, label "AI Sessions". `AppRoot.kt` is the navigation
   `when`; `Api.kt`/`EventStream.kt` the REST + SSE clients; `Events.kt` the
   event model mirror; `ServerConfig.kt` settings + Keystore-sealed token;
   screens in `SessionListScreen/SessionScreen/SpawnScreen/SettingsScreen`.
+- `.dev-updater.ron` — what Dev Updater is asked to do with this checkout:
+  the backend (built in `server/`, installed and controlled through
+  `server/service`) and then the APK (built in `app/`), in that order. The
+  project it serves is the repository, not either half of it, which is why
+  this sits at the root rather than in `app/`.
 - `server/src/certs.rs` — the TLS certificates, generated in process on
   first start into `$XDG_CONFIG_HOME/ai-app/certs`: idempotent CA, leaf
   reissued every start covering every local IPv4 plus 127.0.0.1 and
@@ -121,7 +127,7 @@ Established 2026-08-25, and it decides more than it looks like:
   (qemu `hostfwd`) — usermode networking has none by default.
 - **Nothing secret goes in the repo.** The VM is treated as untrusted (see
   PLAN.md's security section), and the repo is shared read-write with the
-  host, so state lives outside it: `$XDG_CONFIG_HOME/ai-app/config.json`
+  host, so state lives outside it: `$XDG_CONFIG_HOME/ai-app/config.ron`
   and `certs/`, `$XDG_DATA_HOME/ai-app/sessions/`, owner-only.
 - Certificates are generated **by the server, on first start**, into
   `$XDG_CONFIG_HOME/ai-app/certs` (`--certs` overrides). The CA is created
@@ -130,7 +136,7 @@ Established 2026-08-25, and it decides more than it looks like:
   separate throwaway dev CA for emulator work — never install a build
   pinning that on the real phone.
 - Point development at a scratch state directory rather than the real one:
-  `--config /tmp/…/config.json --data-dir /tmp/…/sessions --port 8444`, or
+  `--config /tmp/…/config.ron --data-dir /tmp/…/sessions --port 8444`, or
   `XDG_CONFIG_HOME=… XDG_DATA_HOME=…`.
 
 ## Things that have bitten