Preserve background terminals on interrupt and rename cleanup command to /stop (#14602)

### Motivation
- Interrupting a running turn (Ctrl+C / Esc) currently also terminates
long‑running background shells, which is surprising for workflows like
local dev servers or file watchers.
- The existing cleanup command name was confusing; callers expect an
explicit command to stop background terminals rather than a UI clear
action.
- Make background‑shell termination explicit and surface a clearer
command name while preserving backward compatibility.

### Description
- Renamed the background‑terminal cleanup slash command from `Clean`
(`/clean`) to `Stop` (`/stop`) and kept `clean` as an alias in the
command parsing/visibility layer, updated the user descriptions and
command popup wiring accordingly.
- Updated the unified‑exec footer text and snapshots to point to `/stop`
(and trimmed corresponding snapshot output to match the new label).
- Changed interrupt behavior so `Op::Interrupt` (Ctrl+C / Esc interrupt)
no longer closes or clears tracked unified exec / background terminal
processes in the TUI or core cleanup path; background shells are now
preserved after an interrupt.
- Updated protocol/docs to clarify that `turn/interrupt` (or
`Op::Interrupt`) interrupts the active turn but does not terminate
background terminals, and that `thread/backgroundTerminals/clean` is the
explicit API to stop those shells.
- Updated unit/integration tests and insta snapshots in the TUI and core
unified‑exec suites to reflect the new semantics and command name.

### Testing
- Ran formatting with `just fmt` in `codex-rs` (succeeded). 
- Ran `cargo test -p codex-protocol` (succeeded). 
- Attempted `cargo test -p codex-tui` but the build could not complete
in this environment due to a native build dependency that requires
`libcap` development headers (the `codex-linux-sandbox` vendored build
step); install `libcap-dev` / make `libcap.pc` available in
`PKG_CONFIG_PATH` to run the TUI test suite locally.
- Updated and accepted the affected `insta` snapshots for the TUI
changes so visual diffs reflect the new `/stop` wording and preserved
interrupt behavior.

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_69b39c44b6dc8323bd133ae206310fae)

codex-rs/tui/src/slash_command.rs

@@ -48,7 +48,8 @@ pub enum SlashCommand {
     Feedback,
     Rollout,
     Ps,
-    Clean,
+    #[strum(to_string = "stop", serialize = "clean")]
+    Stop,
     Clear,
     Personality,
     Realtime,
@@ -87,7 +88,7 @@ impl SlashCommand {
             SlashCommand::Statusline => "configure which items appear in the status line",
             SlashCommand::Theme => "choose a syntax highlighting theme",
             SlashCommand::Ps => "list background terminals",
-            SlashCommand::Clean => "stop all background terminals",
+            SlashCommand::Stop => "stop all background terminals",
             SlashCommand::MemoryDrop => "DO NOT USE",
             SlashCommand::MemoryUpdate => "DO NOT USE",
             SlashCommand::Model => "choose what model and reasoning effort to use",
@@ -162,7 +163,7 @@ impl SlashCommand {
             | SlashCommand::Status
             | SlashCommand::DebugConfig
             | SlashCommand::Ps
-            | SlashCommand::Clean
+            | SlashCommand::Stop
             | SlashCommand::Mcp
             | SlashCommand::Apps
             | SlashCommand::Feedback
@@ -196,3 +197,21 @@ pub fn built_in_slash_commands() -> Vec<(&'static str, SlashCommand)> {
         .map(|c| (c.command(), c))
         .collect()
 }
+
+#[cfg(test)]
+mod tests {
+    use pretty_assertions::assert_eq;
+    use std::str::FromStr;
+
+    use super::SlashCommand;
+
+    #[test]
+    fn stop_command_is_canonical_name() {
+        assert_eq!(SlashCommand::Stop.command(), "stop");
+    }
+
+    #[test]
+    fn clean_alias_parses_to_stop_command() {
+        assert_eq!(SlashCommand::from_str("clean"), Ok(SlashCommand::Stop));
+    }
+}