Add an experimental plan tool (#1726)

This adds a tool the model can call to update a plan. The tool doesn't
actually _do_ anything but it gives clients a chance to read and render
the structured plan. We will likely iterate on the prompt and tools
exposed for planning over time.

codex-rs/mcp-server/src/codex_tool_config.rs

@@ -50,6 +50,10 @@ pub struct CodexToolCallParam {
     /// The set of instructions to use instead of the default ones.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub base_instructions: Option<String>,
+
+    /// Whether to include the plan tool in the conversation.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub include_plan_tool: Option<bool>,
 }
 
 /// Custom enum mirroring [`AskForApproval`], but has an extra dependency on
@@ -140,9 +144,10 @@ impl CodexToolCallParam {
             sandbox,
             config: cli_overrides,
             base_instructions,
+            include_plan_tool,
         } = self;
 
-        // Build the `ConfigOverrides` recognised by codex-core.
+        // Build the `ConfigOverrides` recognized by codex-core.
         let overrides = codex_core::config::ConfigOverrides {
             model,
             config_profile: profile,
@@ -152,6 +157,7 @@ impl CodexToolCallParam {
             model_provider: None,
             codex_linux_sandbox_exe,
             base_instructions,
+            include_plan_tool,
         };
 
         let cli_overrides = cli_overrides
@@ -262,6 +268,10 @@ mod tests {
                 "description": "Working directory for the session. If relative, it is resolved against the server process's current working directory.",
                 "type": "string"
               },
+              "include-plan-tool": {
+                "description": "Whether to include the plan tool in the conversation.",
+                "type": "boolean"
+              },
               "model": {
                 "description": "Optional override for the model name (e.g. \"o3\", \"o4-mini\").",
                 "type": "string"