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/mcpd/src/services/instance.service.ts

@@ -32,6 +32,15 @@ export class InstanceService {
     const server = await this.serverRepo.findById(serverId);
     if (!server) throw new NotFoundError(`McpServer '${serverId}' not found`);
 
+    // External servers don't need container management
+    if (server.externalUrl) {
+      return this.instanceRepo.create({
+        serverId,
+        status: 'RUNNING',
+        metadata: { external: true, url: server.externalUrl },
+      });
+    }
+
     const image = server.dockerImage ?? server.packageName ?? server.name;
 
     // Create DB record first in STARTING state
@@ -51,7 +60,11 @@ export class InstanceService {
         },
       };
       if (server.transport === 'SSE' || server.transport === 'STREAMABLE_HTTP') {
-        spec.containerPort = 3000;
+        spec.containerPort = server.containerPort ?? 3000;
+      }
+      const command = server.command as string[] | null;
+      if (command) {
+        spec.command = command;
       }
       if (opts?.env) {
         spec.env = opts.env;