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>

Makefile

@@ -1,11 +1,12 @@
 # CHORUS Multi-Binary Makefile
-# Builds both chorus-agent and chorus-hap binaries
+# Builds chorus-agent, chorus-hap, and seqthink-wrapper binaries
 
 # Build configuration
 BINARY_NAME_AGENT = chorus-agent
 BINARY_NAME_HAP = chorus-hap
 BINARY_NAME_COMPAT = chorus
-VERSION ?= 0.5.5
+BINARY_NAME_SEQTHINK = seqthink-wrapper
+VERSION ?= 0.5.28
 COMMIT_HASH ?= $(shell git rev-parse --short HEAD 2>/dev/null || echo "unknown")
 BUILD_DATE ?= $(shell date -u '+%Y-%m-%d_%H:%M:%S')
 
@@ -49,6 +50,14 @@ build-compat:
 	go build $(BUILD_FLAGS) -o $(BUILD_DIR)/$(BINARY_NAME_COMPAT) ./$(CMD_DIR)/chorus
 	@echo "✅ Compatibility wrapper built: $(BUILD_DIR)/$(BINARY_NAME_COMPAT)"
 
+# Build Sequential Thinking age-encrypted wrapper
+.PHONY: build-seqthink
+build-seqthink:
+	@echo "🔐 Building Sequential Thinking wrapper..."
+	@mkdir -p $(BUILD_DIR)
+	GOWORK=off go build -mod=mod $(BUILD_FLAGS) -o $(BUILD_DIR)/$(BINARY_NAME_SEQTHINK) ./$(CMD_DIR)/seqthink-wrapper
+	@echo "✅ SeqThink wrapper built: $(BUILD_DIR)/$(BINARY_NAME_SEQTHINK)"
+
 # Test compilation without building
 .PHONY: test-compile
 test-compile:
@@ -103,8 +112,13 @@ docker-hap:
 	@echo "🐳 Building Docker image for CHORUS HAP..."
 	docker build -f docker/Dockerfile.hap -t chorus-hap:$(VERSION) .
 
+.PHONY: docker-seqthink
+docker-seqthink:
+	@echo "🔐 Building Docker image for Sequential Thinking wrapper..."
+	docker build -f deploy/seqthink/Dockerfile -t seqthink-wrapper:$(VERSION) .
+
 .PHONY: docker
-docker: docker-agent docker-hap
+docker: docker-agent docker-hap docker-seqthink
 
 # Help
 .PHONY: help
@@ -112,22 +126,24 @@ help:
 	@echo "CHORUS Multi-Binary Build System"
 	@echo ""
 	@echo "Targets:"
-	@echo "  all            - Clean and build both binaries (default)"
-	@echo "  build          - Build both binaries"
+	@echo "  all            - Clean and build all binaries (default)"
+	@echo "  build          - Build all binaries"
 	@echo "  build-agent    - Build autonomous agent binary only"
 	@echo "  build-hap      - Build human agent portal binary only"
-	@echo "  test-compile   - Test that both binaries compile"
+	@echo "  build-seqthink - Build Sequential Thinking wrapper only"
+	@echo "  test-compile   - Test that binaries compile"
 	@echo "  test           - Run tests"
 	@echo "  clean          - Remove build artifacts"
 	@echo "  install        - Install binaries to GOPATH/bin"
 	@echo "  run-agent      - Build and run agent"
 	@echo "  run-hap        - Build and run HAP"
-	@echo "  docker         - Build Docker images for both binaries"
+	@echo "  docker         - Build Docker images for all binaries"
 	@echo "  docker-agent   - Build Docker image for agent only"
 	@echo "  docker-hap     - Build Docker image for HAP only"
+	@echo "  docker-seqthink - Build Docker image for SeqThink wrapper only"
 	@echo "  help           - Show this help"
 	@echo ""
 	@echo "Environment Variables:"
-	@echo "  VERSION        - Version string (default: 0.1.0-dev)"
+	@echo "  VERSION        - Version string (default: 0.5.28)"
 	@echo "  COMMIT_HASH    - Git commit hash (auto-detected)"
-	@echo "  BUILD_DATE     - Build timestamp (auto-generated)"
+	@echo "  BUILD_DATE     - Build timestamp (auto-generated)"