feat: POSIX unification and snapshot sessions (#3179)

## Session snapshot
For POSIX shell, the goal is to take a snapshot of the interactive shell
environment, store it in a session file located in `.codex/` and only
source this file for every command that is run.
As a result, if a snapshot files exist, `bash -lc <CALL>` get replaced
by `bash -c <CALL>`.

This also fixes the issue that `bash -lc` does not source `.bashrc`,
resulting in missing env variables and aliases in the codex session.
## POSIX unification
Unify `bash` and `zsh` shell into a POSIX shell. The rational is that
the tool will not use any `zsh` specific capabilities.

---------

Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/core/src/codex.rs

@@ -405,7 +405,7 @@ impl Session {
         let rollout_fut = RolloutRecorder::new(&config, rollout_params);
 
         let mcp_fut = McpConnectionManager::new(config.mcp_servers.clone());
-        let default_shell_fut = shell::default_user_shell();
+        let default_shell_fut = shell::default_user_shell(conversation_id.0, &config.codex_home);
         let history_meta_fut = crate::message_history::history_metadata(&config);
 
         // Join all independent futures.
@@ -477,6 +477,7 @@ impl Session {
             shell_environment_policy: config.shell_environment_policy.clone(),
             cwd,
         };
+
         let sess = Arc::new(Session {
             conversation_id,
             tx_event: tx_event.clone(),
@@ -2323,13 +2324,25 @@ pub struct ExecInvokeArgs<'a> {
     pub stdout_stream: Option<StdoutStream>,
 }
 
+fn should_translate_shell_command(
+    shell: &crate::shell::Shell,
+    shell_policy: &ShellEnvironmentPolicy,
+) -> bool {
+    matches!(shell, crate::shell::Shell::PowerShell(_))
+        || shell_policy.use_profile
+        || matches!(
+            shell,
+            crate::shell::Shell::Posix(shell) if shell.shell_snapshot.is_some()
+        )
+}
+
 fn maybe_translate_shell_command(
     params: ExecParams,
     sess: &Session,
     turn_context: &TurnContext,
 ) -> ExecParams {
-    let should_translate = matches!(sess.user_shell, crate::shell::Shell::PowerShell(_))
-        || turn_context.shell_environment_policy.use_profile;
+    let should_translate =
+        should_translate_shell_command(&sess.user_shell, &turn_context.shell_environment_policy);
 
     if should_translate
         && let Some(command) = sess
@@ -2946,10 +2959,15 @@ fn convert_call_tool_result_to_function_call_output_payload(
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::config_types::ShellEnvironmentPolicyInherit;
     use mcp_types::ContentBlock;
     use mcp_types::TextContent;
     use pretty_assertions::assert_eq;
     use serde_json::json;
+    use shell::ShellSnapshot;
+    use std::collections::HashMap;
+    use std::path::PathBuf;
+    use std::sync::Arc;
     use std::time::Duration as StdDuration;
 
     fn text_block(s: &str) -> ContentBlock {
@@ -2960,6 +2978,48 @@ mod tests {
         })
     }
 
+    fn shell_policy_with_profile(use_profile: bool) -> ShellEnvironmentPolicy {
+        ShellEnvironmentPolicy {
+            inherit: ShellEnvironmentPolicyInherit::All,
+            ignore_default_excludes: false,
+            exclude: Vec::new(),
+            r#set: HashMap::new(),
+            include_only: Vec::new(),
+            use_profile,
+        }
+    }
+
+    fn zsh_shell(shell_snapshot: Option<Arc<ShellSnapshot>>) -> shell::Shell {
+        shell::Shell::Posix(shell::PosixShell {
+            shell_path: "/bin/zsh".to_string(),
+            rc_path: "/Users/example/.zshrc".to_string(),
+            shell_snapshot,
+        })
+    }
+
+    #[test]
+    fn translates_commands_when_shell_policy_requests_profile() {
+        let policy = shell_policy_with_profile(true);
+        let shell = zsh_shell(None);
+        assert!(should_translate_shell_command(&shell, &policy));
+    }
+
+    #[test]
+    fn translates_commands_for_zsh_with_snapshot() {
+        let policy = shell_policy_with_profile(false);
+        let shell = zsh_shell(Some(Arc::new(ShellSnapshot::new(PathBuf::from(
+            "/tmp/snapshot",
+        )))));
+        assert!(should_translate_shell_command(&shell, &policy));
+    }
+
+    #[test]
+    fn bypasses_translation_for_zsh_without_snapshot_or_profile() {
+        let policy = shell_policy_with_profile(false);
+        let shell = zsh_shell(None);
+        assert!(!should_translate_shell_command(&shell, &policy));
+    }
+
     #[test]
     fn prefers_structured_content_when_present() {
         let ctr = CallToolResult {