feat: experimental menu (#8071)

This will automatically render any `Stage::Beta` features.

The change only gets applied to the *next session*. This started as a
bug but actually this is a good thing to prevent out of distribution
push

<img width="986" height="288" alt="Screenshot 2025-12-15 at 15 38 35"
src="https://github.com/user-attachments/assets/78b7a71d-0e43-4828-a118-91c5237909c7"
/>


<img width="509" height="109" alt="Screenshot 2025-12-15 at 17 35 44"
src="https://github.com/user-attachments/assets/6933de52-9b66-4abf-b58b-a5f26d5747e2"
/>

codex-rs/tui/src/slash_command.rs

@@ -14,6 +14,7 @@ pub enum SlashCommand {
     // more frequently used commands should be listed first.
     Model,
     Approvals,
+    Experimental,
     Skills,
     Review,
     New,
@@ -51,6 +52,7 @@ impl SlashCommand {
             SlashCommand::Status => "show current session configuration and token usage",
             SlashCommand::Model => "choose what model and reasoning effort to use",
             SlashCommand::Approvals => "choose what Codex can do without approval",
+            SlashCommand::Experimental => "toggle beta features",
             SlashCommand::Mcp => "list configured MCP tools",
             SlashCommand::Logout => "log out of Codex",
             SlashCommand::Rollout => "print the rollout file path",
@@ -74,6 +76,7 @@ impl SlashCommand {
             | SlashCommand::Undo
             | SlashCommand::Model
             | SlashCommand::Approvals
+            | SlashCommand::Experimental
             | SlashCommand::Review
             | SlashCommand::Logout => false,
             SlashCommand::Diff