Generalize CodexAgentBackend — it's not actually codex-specific #234

CodexAgentBackend (src/openrange/agent_backend.py) reads like a second Codex backend sitting next to CodexBackend (src/openrange/llm.py), but it has no codex-specific code. It just wraps an LLMBackend into a single-shot, tool-less AgentBackend and defaults to CodexBackend when you don't pass one.

So the "Codex" in the name is misleading — the only codex-specific code is in CodexBackend. Worth renaming/generalizing it (e.g. LLMAgentBackend or SingleShotAgentBackend) so it clearly takes any LLMBackend and "Codex" appears in exactly one place.

Low priority, just a cleanup note.