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>

pkg/seqthink/observability/logger.go

@@ -0,0 +1,39 @@
+package observability
+
+import (
+	"os"
+	"strings"
+	"time"
+
+	"github.com/rs/zerolog"
+	"github.com/rs/zerolog/log"
+)
+
+// InitLogger initializes the global logger
+func InitLogger(level string) {
+	// Set up zerolog with human-friendly console output
+	output := zerolog.ConsoleWriter{
+		Out:        os.Stdout,
+		TimeFormat: time.RFC3339,
+	}
+
+	log.Logger = zerolog.New(output).
+		With().
+		Timestamp().
+		Caller().
+		Logger()
+
+	// Set log level
+	switch strings.ToLower(level) {
+	case "debug":
+		zerolog.SetGlobalLevel(zerolog.DebugLevel)
+	case "info":
+		zerolog.SetGlobalLevel(zerolog.InfoLevel)
+	case "warn":
+		zerolog.SetGlobalLevel(zerolog.WarnLevel)
+	case "error":
+		zerolog.SetGlobalLevel(zerolog.ErrorLevel)
+	default:
+		zerolog.SetGlobalLevel(zerolog.InfoLevel)
+	}
+}