tui: retire /approvals and rename /autoreview to /approve (#21034)

## Why

The TUI currently exposes overlapping command names for the same
permissions flow: `/permissions` and the older `/approvals` alias. It
also uses `/autoreview` for the manual retry flow, even though the
action users take there is approving one denied auto-review request.

This change makes the command surface consistent with the hard rebrand:
- `/permissions` is the only command for permission settings.
- `/approve` is the command for approving a recent auto-review denial.

## What changed

- Removed the legacy `/approvals` slash command and its dispatch path.
- Kept `/permissions` as the single permissions command shown and
accepted by the TUI.
- Renamed the auto-review denial command from `/autoreview` to
`/approve`.
- Updated nearby comments so they refer to `/permissions` rather than
the retired `/approvals` name.

## Verification

- Updated the slash-command unit test to assert that `AutoReview` now
renders and parses as `approve`.

codex-rs/tui/src/chatwidget/slash_dispatch.rs

@@ -237,9 +237,6 @@ impl ChatWidget {
             SlashCommand::Agent | SlashCommand::MultiAgents => {
                 self.app_event_tx.send(AppEvent::OpenAgentPicker);
             }
-            SlashCommand::Approvals => {
-                self.open_permissions_popup();
-            }
             SlashCommand::Permissions => {
                 self.open_permissions_popup();
             }
@@ -890,7 +887,6 @@ impl ChatWidget {
             | SlashCommand::Keymap
             | SlashCommand::Agent
             | SlashCommand::MultiAgents
-            | SlashCommand::Approvals
             | SlashCommand::Permissions
             | SlashCommand::ElevateSandbox
             | SlashCommand::SandboxReadRoot