[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/lib.rs

@@ -39,9 +39,6 @@ pub mod exec_env;
 mod exec_policy;
 #[cfg(test)]
 mod git_info_tests;
-mod goals;
-pub use goals::ExternalGoalPreviousStatus;
-pub use goals::ExternalGoalSet;
 mod guardian;
 mod hook_runtime;
 mod installation_id;