feat: copy tweaks (#2502)

- For selectable options, use sentences starting in lowercase and not
ending with periods. To be honest I don't love this style, but better to
be consistent for now.
- Tweak some other strings.
- Put in more compelling suggestions on launch. Excited to put `/mcp` in
there next.

codex-rs/tui/src/slash_command.rs

@@ -38,7 +38,7 @@ 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::Model => "choose a model preset (model + reasoning effort)",
+            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",
             SlashCommand::Logout => "log out of Codex",