feat(tui): add /statusline command for interactive status line configuration (#10546)

## Summary
- Adds a new `/statusline` command to configure TUI footer status line
- Introduces reusable `MultiSelectPicker` component with keyboard
navigation, optional ordering and toggle support
- Implement status line setup modal that persist configuration to
config.toml

  ## Status Line Items
  The following items can be displayed in the status line:
  - **Model**: Current model name (with optional reasoning level)
  - **Context**: Remaining/used context window percentage
  - **Rate Limits**: 5-day and weekly usage limits
  - **Git**: Current branch (with optimized lookups)
  - **Tokens**: Used tokens, input/output token counts
  - **Session**: Session ID (full or shortened prefix)
  - **Paths**: Current directory, project root
  - **Version**: Codex version

  ## Features
  - Live preview while configuring status line items
  - Fuzzy search filtering in the picker
  - Intelligent truncation when items don't fit
  - Items gracefully omit when data is unavailable
  - Configuration persists to `config.toml`
  - Validates and warns about invalid status line items

  ## Test plan
  - [x] Run `/statusline` and verify picker UI appears
  - [x] Toggle items on/off and verify live preview updates
  - [x] Confirm selection persists after restart
  - [x] Verify truncation behavior with many items selected
  - [x] Test git branch detection in and out of git repos

---------

Co-authored-by: Josh McKinney <joshka@openai.com>

codex-rs/tui/src/slash_command.rs

@@ -34,6 +34,7 @@ pub enum SlashCommand {
     Mention,
     Status,
     DebugConfig,
+    Statusline,
     Mcp,
     Apps,
     Logout,
@@ -65,6 +66,7 @@ impl SlashCommand {
             SlashCommand::Skills => "use skills to improve how Codex performs specific tasks",
             SlashCommand::Status => "show current session configuration and token usage",
             SlashCommand::DebugConfig => "show config layers and requirement sources for debugging",
+            SlashCommand::Statusline => "configure which items appear in the status line",
             SlashCommand::Ps => "list background terminals",
             SlashCommand::Model => "choose what model and reasoning effort to use",
             SlashCommand::Personality => "choose a communication style for Codex",
@@ -131,6 +133,7 @@ impl SlashCommand {
             SlashCommand::TestApproval => true,
             SlashCommand::Collab => true,
             SlashCommand::Agent => true,
+            SlashCommand::Statusline => false,
         }
     }