refactor: remove $-prefixed shell execution in tui

[minorcell]

Summary

• remove direct $ shell execution path from TUI input handling
• remove /$ slash command handling for shell command execution
• update help text and user docs to remove $ shell command guidance
• add CLI regression tests for slash command behavior

Validation

• pnpm run test:cli
• pnpm run build
• pnpm exec prettier --check packages/cli/src/tui/App.tsx packages/cli/src/tui/commands.ts packages/cli/src/tui/slash/help_text.ts packages/cli/src/tui/commands.test.ts docs/user/cli-tui.md