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>

.changeset/config.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://unpkg.com/@changesets/config@3.1.3/schema.json",
+  "changelog": "@changesets/cli/changelog",
+  "commit": false,
+  "fixed": [["@paperclipai/*", "paperclipai"]],
+  "linked": [],
+  "access": "public",
+  "baseBranch": "master",
+  "updateInternalDependencies": "patch",
+  "ignore": ["@paperclipai/ui"]
+}