[codex] TUI leader-key presets and configurable keymaps

[windoliver]

Summary

Implements #186 by replacing the TUI's flat key handling with a leader-key keymap system that supports presets, user overrides, discoverable help, and status-bar hints.

What changed

• Added a strict keymap resolver with sequence support, preset merging, user overrides, and config reload handling.
• Added default and power-user keymap presets.
• Routed TUI keyboard handling, running-view commands, help overlay rows, and status-bar hints through the resolved keymap instead of hardcoded key labels.
• Updated first-run/customize flows so users can choose and preview keymap presets.
• Added focused test coverage for resolver behavior, override loading, help/status rendering, running-view keyboard actions, and preset selection.

Validation

• bun run typecheck
• bun run check
• bun run build
• git diff --check
• Targeted bun test suite: 364 pass / 0 fail; command exits 1 because the repo coverage thresholds remain unmet by the targeted subset.
• Live tmux TUI E2E: codex -> claude-code review-loop handoff completed, with session completion and Nexus VFS records verified.
• Pre-push lefthook reran typecheck and build successfully.

Closes #186