feat(tui) /personality (#9718)

## Summary
Adds /personality selector in the TUI, which leverages the new core
interface in #9644

Notes:
- We are doing some of our own state management for model_info loading
here, but not sure if that's ideal. open to opinions on simpler
approach, but would like to avoid blocking on a larger refactor
- Right now, the `/personality` selector just hides when the model
doesn't support it. we can update this behavior down the line

## Testing
- [x] Tested locally
- [x] Added snapshot tests

codex-rs/tui/src/slash_command.rs

@@ -13,6 +13,7 @@ pub enum SlashCommand {
     // DO NOT ALPHA-SORT! Enum order is presentation order in the popup, so
     // more frequently used commands should be listed first.
     Model,
+    Personality,
     Approvals,
     Permissions,
     #[strum(serialize = "setup-elevated-sandbox")]
@@ -60,6 +61,7 @@ impl SlashCommand {
             SlashCommand::Status => "show current session configuration and token usage",
             SlashCommand::Ps => "list background terminals",
             SlashCommand::Model => "choose what model and reasoning effort to use",
+            SlashCommand::Personality => "choose a communication style for responses",
             SlashCommand::Collab => "change collaboration mode (experimental)",
             SlashCommand::Agent => "switch the active agent thread",
             SlashCommand::Approvals => "choose what Codex can do without approval",
@@ -89,6 +91,7 @@ impl SlashCommand {
             | SlashCommand::Compact
             // | SlashCommand::Undo
             | SlashCommand::Model
+            | SlashCommand::Personality
             | SlashCommand::Approvals
             | SlashCommand::Permissions
             | SlashCommand::ElevateSandbox