Add comprehensive Thruster + HTTP/2 architecture documentation

This commit adds a "Key Learnings" section to the Control Plane readme
documenting the critical insights from deploying Thruster with HTTP/2.

New documentation sections:
1. Protocol Configuration is Critical
   - Explains the common mistake of using protocol: http2
   - Documents why this causes 502 errors

2. Why This Works
   - Visual diagrams comparing standalone vs Control Plane architecture
   - Shows how HTTP/2 is terminated at different points

3. What Thruster Provides on Control Plane
   - Clarifies that Thruster features work with protocol: http
   - Lists all benefits (caching, compression, early hints, etc.)

4. Debugging Tips
   - Commands to verify Thruster is running
   - How to test internal connectivity
   - Where to check protocol settings

This documentation will help future developers avoid the protocol
mismatch issue and understand the correct architecture.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

.controlplane/readme.md

@@ -242,6 +242,50 @@ With Thruster and HTTP/2 enabled on Control Plane, you should see:
 
 For detailed Thruster documentation, see [docs/thruster.md](../docs/thruster.md).
 
+### Key Learnings: Thruster + HTTP/2 Architecture
+
+This section documents important insights gained from deploying Thruster with HTTP/2 on Control Plane.
+
+#### Protocol Configuration is Critical
+
+**Common Mistake:** Setting `protocol: http2` in the workload port configuration
+**Result:** 502 Bad Gateway with "protocol error"
+**Correct Configuration:** Use `protocol: http`
+
+#### Why This Works
+
+Control Plane's architecture differs from standalone Thruster deployments:
+
+**Standalone Thruster (e.g., VPS):**
+```
+User → HTTPS/HTTP2 → Thruster → HTTP/1.1 → Rails
+      (Thruster handles TLS + HTTP/2)
+```
+
+**Control Plane + Thruster:**
+```
+User → HTTPS/HTTP2 → Control Plane LB → HTTP/1.1 → Thruster → HTTP/1.1 → Rails
+                      (LB handles TLS)    (protocol: http)  (HTTP/2 features)
+```
+
+#### What Thruster Provides on Control Plane
+
+Even with `protocol: http`, Thruster still provides:
+- ✅ Asset caching and compression
+- ✅ Efficient static file serving
+- ✅ Early hints support
+- ✅ HTTP/2 multiplexing features (via Control Plane LB)
+
+The HTTP/2 protocol is terminated at Control Plane's load balancer, which then communicates with Thruster via HTTP/1.1. Thruster's caching, compression, and early hints features work regardless of the protocol between the LB and container.
+
+#### Debugging Tips
+
+If you encounter 502 errors:
+1. Verify Thruster is running: `cpln workload exec ... -- cat /proc/1/cmdline`
+2. Test internal connectivity: `cpln workload exec ... -- curl localhost:3000`
+3. Check protocol setting: Should be `protocol: http` not `http2`
+4. Review workload logs: `cpln workload eventlog <workload> --gvc <gvc> --org <org>`
+
 ## Other notes
 
 ### `entrypoint.sh`