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>

src/cli/src/commands/apply.ts

@@ -11,6 +11,9 @@ const ServerSpecSchema = z.object({
   dockerImage: z.string().optional(),
   transport: z.enum(['STDIO', 'SSE', 'STREAMABLE_HTTP']).default('STDIO'),
   repositoryUrl: z.string().url().optional(),
+  externalUrl: z.string().url().optional(),
+  command: z.array(z.string()).optional(),
+  containerPort: z.number().int().min(1).max(65535).optional(),
   envTemplate: z.array(z.object({
     name: z.string(),
     description: z.string().default(''),