Add FS abstraction and use in view_image (#14960)

Adds an environment crate and environment + file system abstraction.

Environment is a combination of attributes and services specific to
environment the agent is connected to:
File system, process management, OS, default shell.

The goal is to move most of agent logic that assumes environment to work
through the environment abstraction.

codex-rs/app-server/src/message_processor.rs

@@ -253,7 +253,7 @@ impl MessageProcessor {
             analytics_events_client,
         );
         let external_agent_config_api = ExternalAgentConfigApi::new(config.codex_home.clone());
-        let fs_api = FsApi;
+        let fs_api = FsApi::default();
 
         Self {
             outgoing,