fix(tui): restore navigation keybind defaults

[kitlangton]

What

Restore the V2 TUI keybind defaults that were unintentionally overwritten or omitted by the command-ID migration in #41882:

• Option+Up / Option+Down switch to the previous / next open tab.
• Option+Shift+Up / Option+Shift+Down switch to the previous / next unread tab.
• Ctrl+0 and <leader>0 select the tenth tab.
• Ctrl+O opens the recent sessions and projects menu.
• Ctrl+Shift+T reopens the last closed tab.
• <leader>I opens image attachments.
• message navigation remains configurable but unbound by default.
• bare Home and End remain available for first/last message navigation rather than prompt-buffer boundaries.

Before / After

Before

The command-ID migration replaced the current tab defaults with stale message-navigation and leader-arrow defaults. It also omitted definitions for commands that remained implemented and registered, so their defaults and migrated user overrides disappeared. Home and End were also rebound to prompt-buffer movement despite #40123 intentionally reserving them for message navigation.

After

The centralized command-ID definitions preserve the effective defaults from immediately before #41882 while retaining the new canonical command IDs.

How

• Restore the affected effective defaults in packages/tui/src/config/keybind.ts using canonical command IDs.
• Add a focused parity test covering every restored default and omitted command.

Scope

This restores confirmed behavioral parity regressions only. It does not remove unrelated dead command definitions introduced by #41882; those can be audited separately without delaying this fix.

Testing

• bun run test test/config-v2.test.tsx test/config.test.tsx test/keymap.test.tsx from packages/tui: 31 passed, 0 failed.
• bunx prettier --check packages/tui/src/config/keybind.ts packages/tui/test/config-v2.test.tsx: passed.
• git diff --check: passed.
• bun typecheck from packages/tui: blocked by unrelated generated-schema skew in the dependency-linked clean worktree (location-switched, AI transport, and system-message schema errors); no errors referenced the changed files.