[codex] Move tool specs into core handlers (#21416)

## Why

This is the first mechanical slice of moving tool spec ownership toward
the handlers. `codex-tools` should keep shared primitives and conversion
helpers, while builtin tool specs and registration planning live in
`codex-core` with the handlers that own those tools.

Keeping this PR to relocation and import updates isolates the copy/move
review from the later logic change that wires specs through registered
handlers.

## What changed

- Moved builtin tool spec constructors from `codex-rs/tools/src` into
`codex-rs/core/src/tools/handlers/*_spec.rs` or nearby core tool
modules.
- Moved the registry planning code into
`codex-rs/core/src/tools/spec_plan.rs` and its associated types/tests
into core.
- Kept shared primitives in `codex-tools`, including `ToolSpec`,
schema/types, discovery/config primitives, dynamic/MCP conversion
helpers, and code-mode collection helpers.
- Updated handlers that referenced moved argument types or tool-name
constants to use the core spec modules.
- Moved spec tests next to the moved spec modules.

## Verification

- `cargo check -p codex-tools`
- `cargo check -p codex-core`
- `cargo test -p codex-tools`
- `cargo test -p codex-core _spec::tests`
- `cargo test -p codex-core tools::spec_plan::tests`
- `just fix -p codex-tools`
- `just fix -p codex-core`

Note: I also tried the broader `cargo test -p codex-core tools::`; it
reached the moved spec-plan/spec tests successfully, then aborted with a
stack overflow in
`tools::handlers::multi_agents::tests::tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtrees_closed`,
which is outside this spec relocation.

codex-rs/core/src/goals.rs

@@ -10,6 +10,7 @@ use crate::session::turn_context::TurnContext;
 use crate::state::ActiveTurn;
 use crate::state::TurnState;
 use crate::tasks::RegularTask;
+use crate::tools::handlers::goal_spec::UPDATE_GOAL_TOOL_NAME;
 use anyhow::Context;
 use codex_features::Feature;
 use codex_otel::GOAL_BUDGET_LIMITED_METRIC;
@@ -317,7 +318,7 @@ impl Session {
                 turn_context,
                 tool_name,
             } => Box::pin(async move {
-                if tool_name != codex_tools::UPDATE_GOAL_TOOL_NAME {
+                if tool_name != UPDATE_GOAL_TOOL_NAME {
                     self.account_thread_goal_progress(
                         turn_context,
                         BudgetLimitSteering::Allowed,