Fix goal update and add /goal edit command in TUI (#21954)

## Why

Users have requested the ability to edit a goal's objective after a goal
has been created. This PR exposes a new `/goal edit` command in the TUI
to address this request.

In the process of implementing this, I also noticed an existing bug in
the goal runtime. When a goal's objective is updated through the
`thread/goal/set` app server API, the goal runtime didn't emit a new
steering prompt to tell the agent about the new objective. This PR also
fixes this hole.

## What Changed

- Adds `/goal edit` in the TUI, opening an edit box prefilled with the
current goal objective.
- Keeps active and paused goals in their current state, resets completed
goals to active, keeps budget-limited goals budget-limited, and
preserves the existing token budget.
- Changes the existing `thread/goal/set` behavior so editing an
objective preserves goal accounting instead of resetting it. The older
reset-on-new-objective behavior was left over from before
`thread/goal/clear`; clients that need to reset accounting can now clear
the existing goal and create a new one.
- Reuses the existing goal set API path; this does not add or change
app-server protocol surface area.
- Adds a dedicated goal runtime steering prompt when an externally
persisted goal mutation changes the objective, so active turns receive
the updated objective.

## Validation

- Make sure `/goal edit` returns an error if no goal currently exists
- Make sure `/goal edit` displays an edit box that can be optionally
canceled with no side effects
- Make sure that an edited goal results in a steer so the agent starts
pursuing the new objective
- Make sure the new objective is reflected in the goal if you use
`/goal` to display the goal summary
- Make sure that `/goal edit` doesn't reset the token budget, time/token
accounting on the updated goal

codex-rs/app-server/src/request_processors/thread_goal_processor.rs

@@ -153,15 +153,13 @@ impl ThreadGoalRequestProcessor {
                 .get_thread_goal(thread_id)
                 .await
                 .map_err(|err| invalid_request(err.to_string()))?;
-            if let Some(goal) = existing_goal.as_ref().filter(|goal| {
-                goal.objective == objective
-                    && goal.status != codex_state::ThreadGoalStatus::Complete
-            }) {
-                let previous_status = ExternalGoalPreviousStatus::Existing(goal.status);
+            if let Some(goal) = existing_goal.as_ref() {
+                let previous_status = ExternalGoalPreviousStatus::from(goal);
                 state_db
                     .update_thread_goal(
                         thread_id,
                         codex_state::ThreadGoalUpdate {
+                            objective: Some(objective.to_string()),
                             status,
                             token_budget: params.token_budget,
                             expected_goal_id: Some(goal.goal_id.clone()),
@@ -198,11 +196,12 @@ impl ThreadGoalRequestProcessor {
                     "cannot update goal for thread {thread_id}: no goal exists"
                 )));
             };
-            let previous_status = ExternalGoalPreviousStatus::Existing(existing_goal.status);
+            let previous_status = ExternalGoalPreviousStatus::from(&existing_goal);
             state_db
                 .update_thread_goal(
                     thread_id,
                     codex_state::ThreadGoalUpdate {
+                        objective: None,
                         status,
                         token_budget: params.token_budget,
                         expected_goal_id: None,