[2 of 2] Finish moving goal runtime to extension (#26548)

## Stack

1. [#26547](https://github.com/openai/codex/pull/26547) - [1 of 2] Align
goal extension with core behavior
2. [#26548](https://github.com/openai/codex/pull/26548) - [2 of 2] Move
goal runtime to extension

## Why

This PR completes the switch of the goal behavior to the
extension-backed runtime and removes the old core goal implementation.

## What Changed

- Installs the goal extension for app-server `ThreadManager` sessions.
- Routes app-server thread goal `get`, `set`, and `clear` through
`GoalService`.
- Uses thread-idle lifecycle emission after goal resume and snapshot
ordering so the extension can decide whether to continue the goal.
- Forwards extension goal updates through a FIFO async app-server
notification path so backpressure does not drop them or reorder updates.
- Keeps review turns from enabling goal runtime behavior.
- Plans extension tools before dynamic tools so built-in goal tool names
keep their old precedence when goals are enabled.
- Removes the old core goal runtime, core goal tool handlers, and core
goal tool specs.
- Updates tests that were coupled to the core-owned goal runtime while
leaving the legacy `<goal_context>` compatibility path in core for old
threads.
- Removes the stale cargo-shear ignore now that `codex-goal-extension`
is used by the workspace.
- Keeps realtime event matching exhaustive after removing the old
goal-specific realtime text path.


## Validation

- Ran manual `/goal` runs in TUI. Validated time accounting matched
wall-clock time and goal lifecycle state transitions.

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

@@ -33,14 +33,14 @@ fn detects_subagent_notification_fragment_case_insensitively() {
 #[test]
 fn detects_internal_model_context_fragment() {
     let text = InternalModelContextFragment::new(
-        InternalContextSource::from_static("goal"),
-        "Continue working toward the active thread goal.",
+        InternalContextSource::from_static("extension"),
+        "Internal steering.",
     )
     .render();
 
     assert_eq!(
         text,
-        "<codex_internal_context source=\"goal\">\nContinue working toward the active thread goal.\n</codex_internal_context>"
+        "<codex_internal_context source=\"extension\">\nInternal steering.\n</codex_internal_context>"
     );
     assert!(is_contextual_user_fragment(&ContentItem::InputText {
         text
@@ -65,7 +65,7 @@ fn does_not_hide_arbitrary_context_tags() {
 #[test]
 fn rejects_invalid_internal_model_context_source() {
     assert!(!is_contextual_user_fragment(&ContentItem::InputText {
-        text: "<codex_internal_context source=\"Goal\">\nbody\n</codex_internal_context>"
+        text: "<codex_internal_context source=\"Extension\">\nbody\n</codex_internal_context>"
             .to_string(),
     }));
 }
@@ -73,13 +73,13 @@ fn rejects_invalid_internal_model_context_source() {
 #[test]
 fn contextual_user_fragment_is_dyn_compatible() {
     let fragment: Box<dyn ContextualUserFragment> = Box::new(InternalModelContextFragment::new(
-        InternalContextSource::from_static("goal"),
-        "Continue working toward the active thread goal.",
+        InternalContextSource::from_static("extension"),
+        "Internal steering.",
     ));
 
     assert_eq!(
         fragment.render(),
-        "<codex_internal_context source=\"goal\">\nContinue working toward the active thread goal.\n</codex_internal_context>"
+        "<codex_internal_context source=\"extension\">\nInternal steering.\n</codex_internal_context>"
     );
 }