Add Codex JSON sessions and usage limits

AGENTS.md

@@ -1,6 +1,6 @@
 # ai-app
 
-A phone interface to AI coding sessions (Claude Code and llama.cpp),
+A phone interface to AI coding sessions (Codex, Claude Code and llama.cpp),
 replacing the Claude app for daily use. Rust/Axum backend on the desktop,
 Kotlin/Compose Android app, WireGuard + pinned self-signed TLS + bearer token
 between them.
@@ -47,6 +47,9 @@ Module-by-module intent is in PLAN.md's "Backend layout".
   readiness poll watches the process as well as the port, since a model that
   will not load exits in a second and was being reported as "gave up after
   300s". See PLAN.md's "Transport" and "llama-server management".
+  Codex is `codex exec --json`, one child process per turn; its driver keeps
+  the thread id for `exec resume`, persists messages queued behind a turn, and
+  reads subscription limits through the CLI's app-server protocol.
 - `app/` — the Compose app, package `com.example.aiapp`, label "AI Sessions".
   `AppRoot.kt` is the navigation `when`; `MainScreen.kt` the root's four tabs
   (sessions, import, models, setups); `Api.kt`/`EventStream.kt` the REST + SSE
@@ -411,4 +414,3 @@ belongs in `~/.claude/TOOLCHAIN.md` or `~/.claude/MACHINE.md` instead.
   shape: the `markdownIn` scan that decides *what* to parse ran before the
   hop to `Dispatchers.Default`. The shape to watch for is a `withContext`
   that wraps the *fetch* and leaves the work done with the result outside it.
-