refactor: remove legacy chat path, fix corrupted conversation recovery

- Delete CodeSession::chat() — the legacy inline tool loop that
  duplicated the orchestrator's conversation + tool dispatch logic
- Delete wait_for_tool_result() — only used by the legacy path
- Make orchestrator mandatory in run_session (no more if/else fallback)
- Unify conversation creation into create_fresh_conversation()
- Add corrupted conversation recovery to create_or_append_conversation:
  detects "function calls and responses" errors from Mistral (caused by
  disconnecting mid-tool-call) and auto-creates a fresh conversation
- Add tracing-appender for optional rotating log file (SOL_LOG_FILE env)
- Add Procfile.dev for overmind process management

Cargo.toml

@@ -22,6 +22,7 @@ serde_json = "1"
 toml = "0.8"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+tracing-appender = "0.2"
 rand = "0.8"
 regex = "1"
 anyhow = "1"