feat: add /ps (#8279)

See snapshots for view of edge cases
This is still named `UnifiedExecSessions` for consistency across the
code but should be renamed to `BackgroundTerminals` in a follow-up

Example:
<img width="945" height="687" alt="Screenshot 2025-12-18 at 20 12 53"
src="https://github.com/user-attachments/assets/92f39ff2-243c-4006-b402-e3fa9e93c952"
/>

codex-rs/tui/src/slash_command.rs

@@ -31,6 +31,7 @@ pub enum SlashCommand {
     Exit,
     Feedback,
     Rollout,
+    Ps,
     TestApproval,
 }
 
@@ -50,6 +51,7 @@ impl SlashCommand {
             SlashCommand::Mention => "mention a file",
             SlashCommand::Skills => "use skills to improve how Codex performs specific tasks",
             SlashCommand::Status => "show current session configuration and token usage",
+            SlashCommand::Ps => "list background terminals",
             SlashCommand::Model => "choose what model and reasoning effort to use",
             SlashCommand::Approvals => "choose what Codex can do without approval",
             SlashCommand::Experimental => "toggle beta features",
@@ -83,6 +85,7 @@ impl SlashCommand {
             | SlashCommand::Mention
             | SlashCommand::Skills
             | SlashCommand::Status
+            | SlashCommand::Ps
             | SlashCommand::Mcp
             | SlashCommand::Feedback
             | SlashCommand::Quit