TUI composer: native Vim modal keymap (opt-in, composer-only)

Summary

Add an opt-in native Vim modal keymap for the TUI composer input box.

Context

Issue #2387 was resolved by external-editor workflow (Ctrl+G), which is useful, but this request is different: native in-composer modal editing for short iterative turns without leaving the input box.

Problem

For terminal-first Vim users, switching to an external editor for many short edits introduces context switches and slows prompt iteration.

Proposal (narrow scope)

• Composer input only (no global/full-app Vim behavior).
• Keep default behavior unchanged (standard).
• Add opt-in vim mode via config and runtime command.
• Show minimal mode indicator in composer UI.
• Non-goal: full Vim parity.

UX / Config

• Config: [tui] keymap = "standard" | "vim" (default standard)
• Runtime: /keymap standard and /keymap vim
• UI: mode indication in composer (and clear normal-mode affordance)

Why this is low risk

• Backward compatible default.
• Explicit opt-in.
• Isolated to composer input behavior.

Prior Art

• Closed issue with external editor approach: #2387
• Implementation reference PR (unsolicited, for design + tests reference): https://github.com/openai/codex/pull/12497

Human + AI calibration notes (for reviewer context)

• Scope was intentionally constrained before coding (composer-only, opt-in).
• Iteration focused on compatibility and testability (default unchanged, explicit command surface, bounded semantics).
• Proposal is ready to be adopted incrementally if maintainers choose to implement internally.