Add goal model tools (3 / 5) (#18075)

Adds the model-facing goal tools on top of the app-server API from PR 2.

## Why

Once goals are persisted and exposed to clients, the model needs a
small, constrained tool surface for goal workflows. The tool contract
should let the model inspect goals, create them only when explicitly
requested, and mark them complete without giving it broad control over
user/runtime-owned state.

## What changed

- Added `get_goal`, `create_goal`, and `update_goal` tool specs behind
the `goals` feature flag.
- Added core goal tool handlers that validate objectives and token
budgets before mutating persisted state.
- Constrained `create_goal` to create only when no goal exists, with
optional `token_budget` only when a budget is explicitly provided.
- Tightened the `create_goal` instructions so the model does not infer
goals from ordinary task requests.
- Constrained `update_goal` to expose only goal completion; pause,
resume, clear, and budget-limited transitions remain user- or
runtime-controlled.
- Registered the goal tools in the tool registry and kept them out of
review contexts where they should not appear.

## Verification

- Added tool-registry coverage for feature gating and tool availability.
- Added core session tests for create/get/update behavior, duplicate
goal rejection, budget validation, and completion-only updates.

codex-rs/tools/src/lib.rs

@@ -6,6 +6,7 @@ mod agent_tool;
 mod apply_patch_tool;
 mod code_mode;
 mod dynamic_tool;
+mod goal_tool;
 mod image_detail;
 mod json_schema;
 mod local_tool;
@@ -51,6 +52,12 @@ pub use code_mode::create_wait_tool;
 pub use code_mode::tool_spec_to_code_mode_tool_definition;
 pub use codex_protocol::ToolName;
 pub use dynamic_tool::parse_dynamic_tool;
+pub use goal_tool::CREATE_GOAL_TOOL_NAME;
+pub use goal_tool::GET_GOAL_TOOL_NAME;
+pub use goal_tool::UPDATE_GOAL_TOOL_NAME;
+pub use goal_tool::create_create_goal_tool;
+pub use goal_tool::create_get_goal_tool;
+pub use goal_tool::create_update_goal_tool;
 pub use image_detail::can_request_original_image_detail;
 pub use image_detail::normalize_output_image_detail;
 pub use image_detail::sanitize_original_image_detail;