Implement Beat 1: Sequential Thinking Age-Encrypted Wrapper (Skeleton)

This commit completes Beat 1 of the SequentialThinkingForCHORUS implementation,
providing a functional plaintext skeleton for the age-encrypted wrapper.

## Deliverables

### 1. Main Wrapper Entry Point
- `cmd/seqthink-wrapper/main.go`: HTTP server on :8443
- Configuration loading from environment variables
- Graceful shutdown handling
- MCP server readiness checking with timeout

### 2. MCP Client Package
- `pkg/seqthink/mcpclient/client.go`: HTTP client for MCP server
- Communicates with MCP server on localhost:8000
- Health check endpoint
- Tool call endpoint with 120s timeout

### 3. Proxy Server Package
- `pkg/seqthink/proxy/server.go`: HTTP handlers for wrapper
- Health and readiness endpoints
- Tool call proxy (plaintext for Beat 1)
- SSE endpoint placeholder
- Metrics endpoint integration

### 4. Observability Package
- `pkg/seqthink/observability/logger.go`: Structured logging with zerolog
- `pkg/seqthink/observability/metrics.go`: Prometheus metrics
- Counters for requests, errors, decrypt/encrypt failures, policy denials
- Request duration histogram

### 5. Docker Infrastructure
- `deploy/seqthink/Dockerfile`: Multi-stage build
- `deploy/seqthink/entrypoint.sh`: Startup orchestration
- `deploy/seqthink/mcp_stub.py`: Minimal MCP server for testing

### 6. Build System Integration
- Updated `Makefile` with `build-seqthink` target
- Uses GOWORK=off and -mod=mod for clean builds
- `docker-seqthink` target for container builds

## Testing

Successfully builds with:
```
make build-seqthink
```

Binary successfully starts and waits for MCP server connection.

## Next Steps

Beat 2 will add:
- Age encryption/decryption (pkg/seqthink/ageio)
- Content-Type: application/age enforcement
- SSE streaming with encrypted frames
- Golden tests for crypto round-trips

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

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

deploy/seqthink/entrypoint.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+set -e
+
+echo "🚀 Starting Sequential Thinking Age Wrapper (Beat 1)"
+
+# Start MCP server on loopback
+echo "📡 Starting MCP server on 127.0.0.1:8000..."
+python3 /opt/mcp/server.py &
+MCP_PID=$!
+
+# Wait for MCP server to be ready
+echo "⏳ Waiting for MCP server to be ready..."
+for i in {1..30}; do
+    if curl -sf http://127.0.0.1:8000/health > /dev/null 2>&1; then
+        echo "✅ MCP server ready"
+        break
+    fi
+    if [ $i -eq 30 ]; then
+        echo "❌ MCP server failed to start"
+        exit 1
+    fi
+    sleep 1
+done
+
+# Start wrapper
+echo "🔐 Starting wrapper on :8443..."
+exec seqthink-wrapper