[prompts] Add <environment_context> (#1869)

## Summary
Includes a new user message in the api payload which provides useful
environment context for the model, so it knows about things like the
current working directory and the sandbox.

## Testing
Updated unit tests

codex-rs/core/src/codex.rs

@@ -37,6 +37,7 @@ use crate::apply_patch::convert_apply_patch_to_protocol;
 use crate::apply_patch::get_writable_roots;
 use crate::apply_patch::{self};
 use crate::client::ModelClient;
+use crate::client_common::EnvironmentContext;
 use crate::client_common::Prompt;
 use crate::client_common::ResponseEvent;
 use crate::config::Config;
@@ -51,6 +52,7 @@ use crate::exec::SandboxType;
 use crate::exec::StdoutStream;
 use crate::exec::process_exec_tool_call;
 use crate::exec_env::create_env;
+use crate::git_info::collect_git_info;
 use crate::mcp_connection_manager::McpConnectionManager;
 use crate::mcp_tool_call::handle_mcp_tool_call;
 use crate::models::ContentItem;
@@ -1224,6 +1226,12 @@ async fn run_turn(
         store: !sess.disable_response_storage,
         tools,
         base_instructions_override: sess.base_instructions.clone(),
+        environment_context: Some(EnvironmentContext {
+            cwd: sess.cwd.clone(),
+            git_info: collect_git_info(&sess.cwd).await,
+            approval_policy: sess.approval_policy,
+            sandbox_policy: sess.sandbox_policy.clone(),
+        }),
     };
 
     let mut retries = 0;
@@ -1449,6 +1457,7 @@ async fn run_compact_task(
         input: turn_input,
         user_instructions: None,
         store: !sess.disable_response_storage,
+        environment_context: None,
         tools: Vec::new(),
         base_instructions_override: Some(compact_instructions.clone()),
     };