fix: point package main to dist entry

[xianzuyang9-blip]

Fixes #3.

The npm package only publishes dist via the files allowlist and the CLI bin already points at dist/index.js, but main still points at a missing root index.js. Clean consumers that import the package root therefore resolve a file that is not included in the tarball.

This updates main to dist/index.js so package-root imports resolve to the bundled entrypoint that is actually published.

Verification:

• Reproduced before the change: packed package installed into a clean consumer, then import('cls-mcp-server') failed with ERR_MODULE_NOT_FOUND for node_modules/cls-mcp-server/index.js.
• Verified after the change: packed package metadata has main: dist/index.js, dist/index.js exists in the installed package, and import('cls-mcp-server') resolves successfully.
• Ran npm test: 5 files / 89 tests passed.

Note: this Windows environment uses Node 24, while the package declares node >=18 <=22, so npm prints an engine warning. The existing npm run build script also invokes Unix chmod; webpack still emitted dist/index.js, but I kept this PR scoped to the package entrypoint bug.