feat: add external MCP server support with streamable-http proxy

Support non-containerized MCP servers via externalUrl field and add
streamable-http session management for HA MCP proof of concept.

- Add externalUrl, command, containerPort fields to McpServer schema
- Skip Docker orchestration for external servers (virtual instances)
- Implement streamable-http proxy with Mcp-Session-Id session management
- Parse SSE-framed responses from streamable-http endpoints
- Add command passthrough to Docker container creation
- Create HA MCP example manifest (examples/ha-mcp.yaml)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

examples/ha-mcp.yaml

@@ -0,0 +1,26 @@
+servers:
+  - name: ha-mcp
+    description: "Home Assistant MCP - smart home control via MCP"
+    dockerImage: "ghcr.io/homeassistant-ai/ha-mcp:2.4"
+    transport: STREAMABLE_HTTP
+    containerPort: 3000
+    # For mcpd-managed containers:
+    command:
+      - python
+      - "-c"
+      - "from ha_mcp.server import HomeAssistantSmartMCPServer; s = HomeAssistantSmartMCPServer(); s.mcp.run(transport='sse', host='0.0.0.0', port=3000)"
+    # For connecting to an already-running instance (host.containers.internal for container-to-host):
+    externalUrl: "http://host.containers.internal:8086/mcp"
+    envTemplate:
+      - name: HOMEASSISTANT_URL
+        description: "Home Assistant instance URL (e.g. https://ha.example.com)"
+      - name: HOMEASSISTANT_TOKEN
+        description: "Home Assistant long-lived access token"
+        isSecret: true
+
+profiles:
+  - name: production
+    server: ha-mcp
+    envOverrides:
+      HOMEASSISTANT_URL: "https://your-ha-instance.example.com"
+      HOMEASSISTANT_TOKEN: "REDACTED-TOKEN"