Tui: Rate limits (#3977)

### /limits: show rate limits graph

<img width="442" height="287" alt="image"
src="https://github.com/user-attachments/assets/3e29a241-a4b0-4df8-bf71-43dc4dd805ca"
/>

### Warning on close to rate limits:

<img width="507" height="96" alt="image"
src="https://github.com/user-attachments/assets/732a958b-d240-4a89-8289-caa92de83537"
/>

Based on #3965

codex-rs/tui/src/slash_command.rs

@@ -21,6 +21,7 @@ pub enum SlashCommand {
     Diff,
     Mention,
     Status,
+    Limits,
     Mcp,
     Logout,
     Quit,
@@ -40,6 +41,7 @@ impl SlashCommand {
             SlashCommand::Diff => "show git diff (including untracked files)",
             SlashCommand::Mention => "mention a file",
             SlashCommand::Status => "show current session configuration and token usage",
+            SlashCommand::Limits => "visualize weekly and hourly rate limits",
             SlashCommand::Model => "choose what model and reasoning effort to use",
             SlashCommand::Approvals => "choose what Codex can do without approval",
             SlashCommand::Mcp => "list configured MCP tools",
@@ -68,6 +70,7 @@ impl SlashCommand {
             SlashCommand::Diff
             | SlashCommand::Mention
             | SlashCommand::Status
+            | SlashCommand::Limits
             | SlashCommand::Mcp
             | SlashCommand::Quit => true,