feat: integrate Changesets for multi-package npm publishing

Migrate from single-bundle CLI publishing to publishing all @paperclipai/*
packages individually via Changesets. This fixes the "Cannot find package
@paperclipai/server" error when installing from npm.

Changes:
- Add @changesets/cli with fixed versioning (all packages share version)
- Make 7 packages publishable (shared, adapter-utils, db, 3 adapters, server)
- Add build scripts, publishConfig, and files fields to all packages
- Mark @paperclipai/server as external in CLI esbuild config
- Simplify CLI importServerEntry() to use string-literal dynamic import
- Add generate-npm-package-json support for external workspace packages
- Create scripts/release.sh for one-command releases
- Remove old bump-and-publish.sh and version-bump.sh
- All packages start at version 0.2.0

Usage: ./scripts/release.sh patch|minor|major [--dry-run]

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

server/package.json

@@ -1,12 +1,29 @@
 {
   "name": "@paperclipai/server",
-  "version": "0.0.1",
-  "private": true,
+  "version": "0.2.0",
   "type": "module",
+  "exports": {
+    ".": "./src/index.ts"
+  },
+  "publishConfig": {
+    "access": "public",
+    "exports": {
+      ".": {
+        "types": "./dist/index.d.ts",
+        "import": "./dist/index.js"
+      }
+    },
+    "main": "./dist/index.js",
+    "types": "./dist/index.d.ts"
+  },
+  "files": [
+    "dist"
+  ],
   "scripts": {
     "dev": "tsx src/index.ts",
     "dev:watch": "PAPERCLIP_MIGRATION_PROMPT=never tsx watch --ignore ../ui/node_modules --ignore ../ui/.vite --ignore ../ui/dist src/index.ts",
     "build": "tsc",
+    "clean": "rm -rf dist",
     "start": "node dist/index.js",
     "typecheck": "tsc --noEmit"
   },