feat(cli): wire --reasoning-effort flag end-to-end — closes ROADMAP #34

Parse --reasoning-effort <low|medium|high> in parse_args, thread through
CliAction::Prompt and CliAction::Repl, LiveCli::set_reasoning_effort(),
AnthropicRuntimeClient.reasoning_effort field, and MessageRequest.reasoning_effort.

Changes:
- parse_args: new --reasoning-effort / --reasoning-effort=VAL flag arms
- AnthropicRuntimeClient: new reasoning_effort field + set_reasoning_effort() method
- LiveCli: new set_reasoning_effort() that reaches through BuiltRuntime -> ConversationRuntime -> api_client_mut()
- runtime::ConversationRuntime: new pub api_client_mut() accessor
- MessageRequest construction: reasoning_effort: self.reasoning_effort.clone()
- run_repl(): accepts and applies reasoning_effort parameter
- parse_direct_slash_cli_action(): propagates reasoning_effort

All 156 CLI tests pass, all api tests pass, cargo fmt clean.

rust/crates/runtime/src/conversation.rs

@@ -504,6 +504,10 @@ where
         &self.session
     }
 
+    pub fn api_client_mut(&mut self) -> &mut C {
+        &mut self.api_client
+    }
+
     pub fn session_mut(&mut self) -> &mut Session {
         &mut self.session
     }