feat(llm): auto-start proxy after setup when proxy-mode tools are configured

Currently thv llm setup detects proxy-mode tools (Cursor, VS Code, Xcode), patches their config files, then exits with a printed reminder:

One or more tools use proxy mode. Start the proxy with: thv llm proxy start

(see pkg/llm/setup.go:141-142)

Users must manually run thv llm proxy start in a separate terminal and keep it running. This is an extra friction step that is easy to miss and causes proxy-mode tools to silently fail.

Proposal: When thv llm setup detects at least one proxy-mode tool, automatically start the proxy process (equivalent to thv llm proxy start) at the end of setup rather than just printing a reminder. This could be:

• Foreground (blocking) — simple, consistent with current thv llm proxy start behavior
• Background daemon — lower friction but adds complexity (PID management, restart on reboot)

The foreground option is the minimal change: after persisting tool configs, if hasProxyMode(configured) is true, call the proxy start logic directly instead of (or in addition to) printing the reminder message.

Notes:

• Token-helper / direct-mode tools (Claude Code, Gemini CLI) do NOT need the proxy and should be unaffected.
• The proxy could still be exposed as a standalone thv llm proxy start command for users who want explicit control or need to restart it independently.

Start at pkg/llm/setup.go:141-142 and trace the existing thv llm proxy start entry point. Determine how setup persists proxy-mode tool configurations and how the chosen foreground or background startup should integrate, while leaving token-helper/direct-mode tools unchanged. Done means proxy-mode tools work immediately after setup and standalone proxy start remains available.