fix: add previous_response_id support for multi-turn tool calls (Crof fix)

Codex Desktop uses previous_response_id to chain conversation turns.
Without storing and resolving these, the proxy sent only the new
function_call_output to upstream providers, missing the original user
message and assistant tool call. This caused Crof.ai (and any provider
using tool calls) to stop after the first response.

- Add in-memory response store (50 entry LRU) keyed by response ID
- resolve_previous_response() reconstructs full input chain on multi-turn
- Fix orphan message output item when response has only tool calls
- Applies to all backends: openai-compat, anthropic, command-code
- v2.1.2

README.md

@@ -390,6 +390,7 @@ README.md                         # This file
 | Proxy stops when terminal closes | SIGHUP to subprocess | Launcher uses `os.setsid` process groups |
 | Models not showing in picker | Wrong model catalog format | Must have both `slug` + `model` fields |
 | Codex hangs in "thinking" | Missing `response.completed` | Proxy emits full SSE event sequence |
+| Stops after first tool call (Crof) | `previous_response_id` not resolved | V2.1.2 stores and chains responses for multi-turn |
 
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