[codex] Support more Vim normal commands

[jinghanx88]

Summary

• Add more Vim normal-mode navigation and operator targets for large composer buffers, including gg/G, dG, yG, dgg, and cG/cgg.
• Support c{motion} for normal motions such as cw, c$, and cc, entering insert mode after deleting the target range.
• Add characterwise visual mode with v and linewise visual mode with V, including y, d/x, and c/s actions on the selected range.
• Add normal-mode repeat with ., multi-level undo with u, and redo with Ctrl-R.
• Expose the new Vim bindings through config/schema and /keymap, and render visual/visual-line indicators without panicking.

Test plan

• just write-config-schema
• just test -p codex-config
• Focused just test -p codex-tui ... coverage for Vim change motions, visual line mode, repeat, undo/redo, and gg/G operator targets.
• just test -p codex-tui keymap_setup::tests after accepting expected keymap action-count snapshots.
• cargo insta pending-snapshots --manifest-path tui/Cargo.toml reports no pending snapshots.
• just test -p codex-tui passes 2772 tests and still fails only the existing Guardian tests: app::tests::update_feature_flags_disabling_guardian_clears_manual_review_policy_without_history, app::tests::update_feature_flags_disabling_guardian_clears_review_policy_and_restores_default.
• just fix -p codex-tui
• git diff --check