Add goal TUI UX (5 / 5) (#18077)

Adds the TUI user experience for goals on top of the core runtime from
PR 4.

## Why

Users need a direct TUI control surface for long-running goals. The UI
should make the current goal visible, support common goal actions
without waiting for a model turn, and avoid confusing end-of-turn
notifications while an active goal is immediately continuing.

## What changed

- Added `/goal` summary rendering for the current goal, including
active, paused, budget-limited, and complete states.
- Added `/goal <objective>` creation/replacement through the app-server
goal API rather than a model prompt.
- Added `/goal clear`, `/goal pause`, and `/goal unpause` command
variants.
- Added a confirmation menu when the user enters a new goal while
another goal already exists.
- Updated `/goal` help and summary tip text so it reflects the supported
command variants without advertising slash-command token budgets.
- Added footer/statusline goal indicators, including elapsed time and
token budget display when a budget exists from API/tool-created goals.
- Consumes goal updated/cleared notifications so the TUI stays in sync
with external app-server changes.
- Suppresses end-of-turn desktop notifications only when a goal is still
active and follow-up work is expected.
- Preserves slash-command history behavior and avoids leaking queued
`/goal` state into unrelated submissions.

## Verification

- Added TUI unit and snapshot coverage for goal command availability,
summary rendering, control commands, replacement menu behavior,
status/footer display, notification handling, and command history.

codex-rs/tui/src/slash_command.rs

@@ -31,6 +31,7 @@ pub enum SlashCommand {
     Init,
     Compact,
     Plan,
+    Goal,
     Collab,
     Agent,
     Side,
@@ -104,6 +105,7 @@ impl SlashCommand {
             SlashCommand::Realtime => "toggle realtime voice mode (experimental)",
             SlashCommand::Settings => "configure realtime microphone/speaker",
             SlashCommand::Plan => "switch to Plan mode",
+            SlashCommand::Goal => "set or view the goal for a long-running task",
             SlashCommand::Collab => "change collaboration mode (experimental)",
             SlashCommand::Agent | SlashCommand::MultiAgents => "switch the active agent thread",
             SlashCommand::Side => "start a side conversation in an ephemeral fork",
@@ -137,6 +139,7 @@ impl SlashCommand {
             SlashCommand::Review
                 | SlashCommand::Rename
                 | SlashCommand::Plan
+                | SlashCommand::Goal
                 | SlashCommand::Fast
                 | SlashCommand::Mcp
                 | SlashCommand::Side
@@ -186,6 +189,7 @@ impl SlashCommand {
             | SlashCommand::DebugConfig
             | SlashCommand::Ps
             | SlashCommand::Stop
+            | SlashCommand::Goal
             | SlashCommand::Mcp
             | SlashCommand::Apps
             | SlashCommand::Plugins
@@ -239,4 +243,9 @@ mod tests {
     fn clean_alias_parses_to_stop_command() {
         assert_eq!(SlashCommand::from_str("clean"), Ok(SlashCommand::Stop));
     }
+
+    #[test]
+    fn goal_command_is_available_during_task() {
+        assert!(SlashCommand::Goal.available_during_task());
+    }
 }