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

PLAN.md

@@ -76,7 +76,7 @@ backend (Rust/Axum, desktop)
    │        decided per session by the host it names
    ├─ LlamaServerManager  (llama-server lifecycle, local + SSH)
    ├─ UsageMonitor        (Anthropic OAuth usage endpoint)
-   └─ config.json + per-session transcript files
+   └─ config.ron + per-session transcript files
 ```
 
 ### Backend layout (`server/`)
@@ -196,7 +196,7 @@ turn. Claude's dialect: a `user` message on stdin mid-stream; pi's: `steer`.
 
 ### llama-server management
 
-`config.json` lists **models** (name → GGUF path or llama-server args, per
+`config.ron` lists **models** (name → GGUF path or llama-server args, per
 host) and **hosts**. The manager runs at most one llama-server per
 `(host, model)`, spawned on demand when a session needs it:
 
@@ -215,7 +215,7 @@ host) and **hosts**. The manager runs at most one llama-server per
 
 ### SSH
 
-- Host entries in `config.json`: name, `user@host`, optional ssh options,
+- Host entries in `config.ron`: name, `user@host`, optional ssh options,
   which capabilities it has (claude / pi / llama-server, with paths if not on
   PATH). Key-based auth only, using the system `ssh` client via
   `tokio::process` — no Rust SSH library; this inherits `~/.ssh/config`,
@@ -267,7 +267,7 @@ GET  /usage                           cached usage windows
 GET/PUT /hosts, /models               config editing from the phone
 ```
 
-Sessions live in `config.json` (`$XDG_CONFIG_HOME/ai-app/`) + a per-session
+Sessions live in `config.ron` (`$XDG_CONFIG_HOME/ai-app/`) + a per-session
 directory under `$XDG_DATA_HOME/ai-app/sessions/` (transcript.jsonl,
 attachments, produced images), owner-only. Deleting a session is the
 complete path out of everything spawning one created.
@@ -300,7 +300,7 @@ complete path out of everything spawning one created.
   attacker-writable. Two consequences:
   - **Nothing secret lives in the repo.** Certificates are generated on
     the machine that serves them and written to
-    `$XDG_CONFIG_HOME/ai-app/certs` (0700, keys 0600); `config.json` and
+    `$XDG_CONFIG_HOME/ai-app/certs` (0700, keys 0600); `config.ron` and
     session transcripts go to the XDG config and data directories, per
     machine. A CA private key the VM could read would let it mint a leaf
     the pinned app accepts, which is precisely the attack pinning exists
@@ -344,7 +344,7 @@ complete path out of everything spawning one created.
     scanner Activity reached through the AndroidX Activity Result API,
     fully offline, no Play Services/ML Kit model download) and feeds the
     decoded URI to the same `parseEnrollmentUri` (2026-08-25).
-  - **Storage**: server keeps only the SHA-256 in `config.json` (plain hash
+  - **Storage**: server keeps only the SHA-256 in `config.ron` (plain hash
     is enough for high-entropy random input; buys that a leaked config
     doesn't leak the credential). No "show token again" — lost means rotate.
     Phone side: sealed with an Android Keystore AES-GCM key (a small
@@ -475,7 +475,7 @@ window just fills.
 
 1. **Skeleton** — *done 2026-08-24.* Repo layout, cert script, TLS + token
    auth, wg0-bound listener (fail closed if the interface is missing),
-   config.json, session registry with a fake `EchoDriver`, session list +
+   config.ron, session registry with a fake `EchoDriver`, session list +
    session screen in the app end-to-end over SSE. Proves the whole pipe
    before any AI is involved. Verified: 10 server tests + clippy clean;
    curl end-to-end over pinned TLS (auth rejection, spawn, SSE