[codex] simplify token budget context (#29295)

## Why

The token-budget feature currently adds remaining-token messages
whenever usage crosses the 25%, 50%, and 75% thresholds. Those periodic
inserts create prompt churn without requiring action, while the
near-compaction reminder and explicit `get_context_remaining` tool
already cover actionable and on-demand budget information.

The context-window lineage block is also easier to scan as plain labeled
text than as a `<token_budget>`-wrapped fragment.

## What changed

- Stop recording automatic remaining-token messages at percentage
thresholds.
- Render context-window lineage in `First`, `Current`, `Previous` order
with colon-separated labels.
- Omit the `Previous` line for the first context window.
- Remove `<token_budget>` wrappers from newly rendered lineage,
near-compaction reminders, and `get_context_remaining` output.
- Keep recognizing legacy wrapped fragments so existing rollouts remain
compatible.
- Remove the post-sampling token snapshot that was only needed by the
periodic threshold path.

## Testing

- `just test -p codex-core token_budget` (11 tests passed)

codex-rs/core/src/context/token_budget_context.rs

@@ -39,24 +39,25 @@ impl ContextualUserFragment for TokenBudgetContext {
     }
 
     fn type_markers() -> (&'static str, &'static str) {
-        ("<token_budget>\n", "\n</token_budget>")
+        ("", "")
     }
 
     fn body(&self) -> String {
         let thread_id = self.thread_id;
         let first_window_id = self.first_window_id;
-        let previous_window_id = self
-            .previous_window_id
-            .map_or_else(|| "none".to_string(), |window_id| window_id.to_string());
         let window_id = self.window_id;
-        let mcp_result = self
-            .mcp_result
-            .as_deref()
-            .map(|result| format!("\n{result}"))
-            .unwrap_or_default();
-        format!(
-            "Thread id {thread_id}.\nFirst context window id {first_window_id}.\nPrevious context window id {previous_window_id}.\nCurrent context window id {window_id}.{mcp_result}"
-        )
+        let mut lines = vec![
+            format!("Thread id {thread_id}."),
+            format!("First context window id: {first_window_id}"),
+            format!("Current context window id: {window_id}"),
+        ];
+        if let Some(previous_window_id) = self.previous_window_id {
+            lines.push(format!("Previous context window id: {previous_window_id}"));
+        }
+        if let Some(mcp_result) = &self.mcp_result {
+            lines.push(mcp_result.clone());
+        }
+        lines.join("\n")
     }
 }
 
@@ -87,7 +88,7 @@ impl ContextualUserFragment for TokenBudgetRemainingContext {
     }
 
     fn type_markers() -> (&'static str, &'static str) {
-        ("<token_budget>\n", "\n</token_budget>")
+        ("", "")
     }
 
     fn body(&self) -> String {
@@ -123,7 +124,7 @@ impl ContextualUserFragment for TokenBudgetReminder {
     }
 
     fn type_markers() -> (&'static str, &'static str) {
-        ("<token_budget>\n", "\n</token_budget>")
+        ("", "")
     }
 
     fn body(&self) -> String {