Add goal lifecycle metrics (#20799)

## Why

Adding goal metrics makes it possible to track how often goals are
created, completed, and stopped by budget limits, plus the final token
and wall-clock usage for terminal outcomes.

## What Changed

- Added OpenTelemetry metric constants for goal lifecycle tracking:
- `codex.goal.created`: increments each time a new persisted goal is
created or an existing goal is replaced with a new objective.
- `codex.goal.completed`: increments when a goal transitions to
`complete`.
- `codex.goal.budget_limited`: increments when a goal transitions to
`budget_limited` because its token budget has been reached.
- `codex.goal.token_count`: records the final persisted token count when
a goal transitions to `complete` or `budget_limited`.
- `codex.goal.duration_s`: records the final persisted elapsed
wall-clock time, in seconds, when a goal transitions to `complete` or
`budget_limited`.
- Emitted creation metrics when a goal is created or replaced.
- Emitted terminal outcome counters and final usage histograms when a
goal transitions to `complete` or `budget_limited`, avoiding
double-counting later in-flight accounting for already budget-limited
goals.
- Added focused `codex-core` tests for create/complete metrics and
one-time budget-limit metrics.

codex-rs/core/src/codex_thread.rs

@@ -1,6 +1,7 @@
 use crate::agent::AgentStatus;
 use crate::config::ConstraintResult;
 use crate::file_watcher::WatchRegistration;
+use crate::goals::ExternalGoalSet;
 use crate::goals::GoalRuntimeEvent;
 use crate::session::Codex;
 use crate::session::SessionSettingsUpdate;
@@ -160,11 +161,11 @@ impl CodexThread {
         }
     }
 
-    pub async fn apply_external_goal_set(&self, status: codex_state::ThreadGoalStatus) {
+    pub async fn apply_external_goal_set(&self, external_set: ExternalGoalSet) {
         if let Err(err) = self
             .codex
             .session
-            .goal_runtime_apply(GoalRuntimeEvent::ExternalSet { status })
+            .goal_runtime_apply(GoalRuntimeEvent::ExternalSet { external_set })
             .await
         {
             tracing::warn!("failed to apply external goal status runtime effects: {err}");