chore: add small debug client (#8894)

Small debug client, do not use in production

codex-rs/debug-client/README.md

@@ -0,0 +1,57 @@
+WARNING: this code is mainly generated by Codex and should not be used in production
+
+# codex-debug-client
+
+A tiny interactive client for `codex app-server` (protocol v2 only). It prints
+all JSON-RPC lines from the server and lets you send new turns as you type.
+
+## Usage
+
+Start the app-server client (it will spawn `codex app-server` itself):
+
+```
+cargo run -p codex-debug-client -- \
+  --codex-bin codex \
+  --approval-policy on-request
+```
+
+You can resume a specific thread:
+
+```
+cargo run -p codex-debug-client -- --thread-id thr_123
+```
+
+### CLI flags
+
+- `--codex-bin <path>`: path to the `codex` binary (default: `codex`).
+- `-c, --config key=value`: pass through `--config` overrides to `codex`.
+- `--thread-id <id>`: resume a thread instead of starting a new one.
+- `--approval-policy <policy>`: `untrusted`, `on-failure`, `on-request`, `never`.
+- `--auto-approve`: auto-approve command/file-change approvals (default: decline).
+- `--final-only`: only show completed assistant messages and tool items.
+- `--model <name>`: optional model override for thread start/resume.
+- `--model-provider <name>`: optional provider override.
+- `--cwd <path>`: optional working directory override.
+
+## Interactive commands
+
+Type a line to send it as a new turn. Commands are prefixed with `:`:
+
+- `:help` show help
+- `:new` start a new thread
+- `:resume <thread-id>` resume a thread
+- `:use <thread-id>` switch active thread without resuming
+- `:refresh-thread` list available threads
+- `:quit` exit
+
+The prompt shows the active thread id. Client messages (help, errors, approvals)
+print to stderr; raw server JSON prints to stdout so you can pipe/record it
+unless `--final-only` is set.
+
+## Notes
+
+- The client performs the required initialize/initialized handshake.
+- It prints every server notification and response line as it arrives.
+- Approvals for `item/commandExecution/requestApproval` and
+  `item/fileChange/requestApproval` are auto-responded to with decline unless
+  `--auto-approve` is set.