feat: agnostic tunnel system — bring your own tunnel provider

New Tunnel trait + 5 implementations:
- NoneTunnel: local-only, no external exposure (default)
- CloudflareTunnel: wraps cloudflared binary, extracts public URL
- TailscaleTunnel: tailscale serve (tailnet) or funnel (public)
- NgrokTunnel: wraps ngrok binary, supports custom domains
- CustomTunnel: user-provided command with {port}/{host} placeholders

Config schema:
- [tunnel] section with provider selector
- Provider-specific sub-configs: cloudflare, tailscale, ngrok, custom
- Backward compatible (serde default = "none")

Gateway integration:
- Tunnel starts automatically on 'zeroclaw gateway'
- Prints public URL on success, falls back to local on failure

20 new tests (factory, constructors, NoneTunnel async start/health)
649 tests passing, 0 clippy warnings, cargo fmt clean

src/main.rs

@@ -28,6 +28,7 @@ mod runtime;
 mod security;
 mod skills;
 mod tools;
+mod tunnel;
 
 use config::Config;