feat: BYOP provider + tunnel wizard + SVG architecture diagram

Custom Provider (Bring Your Own):
- Add custom:URL format to provider factory (any OpenAI-compatible API)
- Works with LiteLLM, LocalAI, vLLM, text-generation-webui, LM Studio, etc.
- Example: default_provider = 'custom:http://localhost:1234'
- 4 new tests for custom provider (URL, localhost, no-key, empty-URL error)

Setup Wizard (6 steps, 5-year-old friendly):
- Add '🔧 Custom' tier to provider selection with guided BYOP flow
- Add Step 4: Tunnel setup (Cloudflare, Tailscale, ngrok, Custom, or skip)
- Emoji labels on all provider categories for visual clarity
- Renumber wizard to 6 steps (was 5)

Architecture Diagram:
- New SVG diagram at docs/architecture.svg (dark theme, color-coded)
- Shows: Chat Apps → Security → Agent Loop → AI Providers
- Shows: Tunnel layer, Sandbox, Context, Heartbeat/Cron
- Shows: Setup Wizard 6-step flow at bottom
- Replace ASCII art in README with SVG embed

657 tests passing, 0 clippy warnings, cargo fmt clean

src/providers/mod.rs

@@ -88,8 +88,24 @@ pub fn create_provider(name: &str, api_key: Option<&str>) -> anyhow::Result<Box<
             "Cohere", "https://api.cohere.com/compatibility", api_key, AuthStyle::Bearer,
         ))),
 
+        // ── Bring Your Own Provider (custom URL) ───────────
+        // Format: "custom:https://your-api.com" or "custom:http://localhost:1234"
+        name if name.starts_with("custom:") => {
+            let base_url = name.strip_prefix("custom:").unwrap_or("");
+            if base_url.is_empty() {
+                anyhow::bail!("Custom provider requires a URL. Format: custom:https://your-api.com");
+            }
+            Ok(Box::new(OpenAiCompatibleProvider::new(
+                "Custom",
+                base_url,
+                api_key,
+                AuthStyle::Bearer,
+            )))
+        }
+
         _ => anyhow::bail!(
-            "Unknown provider: {name}. Run `zeroclaw integrations list -c ai` to see all available providers."
+            "Unknown provider: {name}. Run `zeroclaw integrations list -c ai` to see all available providers.\n\
+             Tip: Use \"custom:https://your-api.com\" for any OpenAI-compatible endpoint."
         ),
     }
 }
@@ -231,6 +247,37 @@ mod tests {
         assert!(create_provider("cohere", Some("key")).is_ok());
     }
 
+    // ── Custom / BYOP provider ─────────────────────────────
+
+    #[test]
+    fn factory_custom_url() {
+        let p = create_provider("custom:https://my-llm.example.com", Some("key"));
+        assert!(p.is_ok());
+    }
+
+    #[test]
+    fn factory_custom_localhost() {
+        let p = create_provider("custom:http://localhost:1234", Some("key"));
+        assert!(p.is_ok());
+    }
+
+    #[test]
+    fn factory_custom_no_key() {
+        let p = create_provider("custom:https://my-llm.example.com", None);
+        assert!(p.is_ok());
+    }
+
+    #[test]
+    fn factory_custom_empty_url_errors() {
+        match create_provider("custom:", None) {
+            Err(e) => assert!(
+                e.to_string().contains("requires a URL"),
+                "Expected 'requires a URL', got: {e}"
+            ),
+            Ok(_) => panic!("Expected error for empty custom URL"),
+        }
+    }
+
     // ── Error cases ──────────────────────────────────────────
 
     #[test]