Why

Picker-style UI in the TUI has accumulated a mix of hardcoded navigation keys. Some lists supported page movement, some did not; some accepted Vim-like keys, while others only accepted arrows; and tabbed or horizontally adjustable pickers had no shared keymap action for left/right movement.

This PR makes picker/list navigation consistent and configurable so users can rely on the same defaults across the TUI.

What Changed

• Adds shared list keymap actions for:
  • vertical movement: move_up, move_down
  • horizontal movement: move_left, move_right
  • paging and jumps: page_up, page_down, jump_top, jump_bottom
• Adds defaults:
  • Up/down: arrows, Ctrl+P/N, Ctrl+K/J, and plain k/j where text input is not active
  • Page up/down: PageUp/PageDown and Ctrl+B/F
  • First/last: Home/End
  • Left/right: Left/Right and Ctrl+H/L
• Wires the shared list keymap through picker and list surfaces including session resume, multi-select, tabbed selection lists, settings-style lists, app-link selection, MCP elicitation, request-user-input, and the OSS selection wizard.
• Keeps search behavior intact by reserving printable characters for query text in searchable pickers.
• Updates keymap setup actions, config schema, snapshots, and focused coverage for the new list actions.

How to Test

1. Start Codex from this branch and open the session picker, for example with an existing session history.
2. In the session list, verify that Ctrl+J/K moves the selection down/up.
3. Verify that Ctrl+F/B pages down/up and Home/End jumps to the first/last visible session.
4. Type printable search text such as j or k and confirm it updates the query instead of navigating.
5. Focus a picker control that changes values horizontally, such as a session picker toolbar control, and verify Ctrl+H/L changes the focused value like left/right arrows.

Targeted tests run:

• cargo test -p codex-tui keymap::tests::
• cargo test -p codex-tui keymap_setup::tests::
• cargo test -p codex-tui horizontal_list_keys
• cargo test -p codex-tui page_and_jump_navigation_use_list_keymap
• cargo test -p codex-tui ctrl_h_l_move_provider_selection
• cargo test -p codex-tui scroll_state::tests
• cargo test -p codex-tui switching_tabs_changes_visible_items_and_clears_search
• cargo test -p codex-tui toggle_sort_key_reloads_with_new_sort

Also ran just write-config-schema, just fmt, just fix -p codex-tui, just argument-comment-lint, and git diff --check.

Note: cargo test -p codex-tui was attempted and still aborts in the pre-existing tests::fork_last_filters_latest_session_by_cwd_unless_show_all stack overflow, which is unrelated to this branch.