feat: ship docker as a first-party in-process plugin

Moves docker backend and frontend into plugins/docker/, alongside
ssh-terminal. Adds a host editor tab seam, a dashboard card registry,
and a build step to compile plugin backends into dist.

tsconfig.plugins.json

@@ -0,0 +1,31 @@
+{
+  "compilerOptions": {
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.plugins.tsbuildinfo",
+    "target": "ES2023",
+    "lib": ["ES2023"],
+    "module": "nodenext",
+    "skipLibCheck": true,
+    "moduleResolution": "nodenext",
+    "verbatimModuleSyntax": true,
+    "moduleDetection": "force",
+    "esModuleInterop": true,
+    "noEmit": false,
+    "rootDir": ".",
+    "outDir": ".",
+    "strict": false,
+    "noUnusedLocals": false,
+    "noUnusedParameters": false,
+    "noImplicitReturns": false,
+    "noFallthroughCasesInSwitch": false,
+    "noUncheckedSideEffectImports": false,
+    "noImplicitAny": false,
+    "noImplicitThis": false,
+    "noUncheckedIndexedAccess": false,
+    "exactOptionalPropertyTypes": false,
+    "noPropertyAccessFromIndexSignature": false,
+    "allowUnusedLabels": true,
+    "allowUnreachableCode": true
+  },
+  "include": ["plugins/docker/backend/**/*.ts"],
+  "exclude": ["src/**/*.test.ts"]
+}