Let the command go to Codex or OpenRouter, not only Claude Code

Everything the last commit built assumed one agent was installed, which is a
poor assumption to bake into a dictation tool. So the provider is a setting, and
what it selects is one of three quite different things.

Claude Code and Codex are the same shape: a CLI, streaming JSONL, a session id
to resume, tools that reach the machine and whatever is connected to it. They
share the runner. What differs is spelled out where it differs, which is more
than the flag names: Codex has no system prompt to append, so the instruction
rides in front of the command with a rule between them; it confines its commands
in a sandbox rather than asking about them, so the permission setting is a
sandbox mode; and `-s` is not accepted by `exec resume`, so both settings go
through `-c` overrides, which are.

OpenRouter is the odd one and is meant to be. No tools, no files, no calendar:
it can say what the capital of Peru is and not what is in your diary, and the
settings box says so rather than letting it be discovered. It also has no
session to resume, so the conversation is kept here and resent, capped at 24
messages.

A stored conversation names the provider that made it, and is ignored by any
other: none of them can pick up another's thread, and a stale id would otherwise
fail every command until the timeout cleared it.

The interface calls the thing by its name, which in Turkish means the suffix has
to agree with it: Claude'a but Codex'e, Claude'u but Codex'i. A name dropped
into a sentence through t() cannot be inflected by that sentence, so it arrives
inflected, from a small table in i18n. English takes the name as it is and keeps
the preposition in the sentence.

api.py

@@ -209,6 +209,38 @@ def cleanup(text, api_key, model, system_prompt, reasoning="",
     return content
 
 
+def chat(messages, api_key, model, system_prompt, base_url=OPENROUTER_URL,
+         timeout=180):
+    """A conversation, rather than one transcript rewritten.
+
+    The messages are the whole history and come back unchanged; the caller keeps
+    them, because there is no session on OpenRouter's side to resume.
+    """
+    if not api_key:
+        raise ApiError(t("{service} API key is empty. Add it in Settings.",
+                         service="OpenRouter"))
+    payload = {
+        "model": model,
+        "messages": [{"role": "system", "content": system_prompt}] + list(messages),
+    }
+    try:
+        data = _request(
+            f"{base_url.rstrip('/')}/chat/completions",
+            json.dumps(payload).encode("utf-8"),
+            _headers("openrouter", api_key, "application/json"),
+            timeout=timeout,
+        )
+    except ApiError as exc:
+        raise explain(exc, "OpenRouter") from None
+    choices = data.get("choices") or []
+    if not choices:
+        raise ApiError(_extract_error(json.dumps(data)))
+    content = ((choices[0].get("message") or {}).get("content") or "").strip()
+    if not content:
+        raise ApiError(t("The model returned an empty reply."))
+    return content
+
+
 def _get_json(url, headers, timeout=20):
     req = urllib.request.Request(url, headers=headers)
     try: