chore: add changesets alpha setup

[mahyarmlk]

Summary

Add Changesets for release management and configure first alpha package versions.

Why

Prepare the repository for the first npm alpha release without publishing anything and without adding the GitHub release workflow yet.

Changes

• Add @changesets/cli as root dev dependency
• Initialize Changesets with .changeset/config.json and .changeset/README.md
• Configure Changesets with access: public, baseBranch: main, privatePackages: { version: false, tag: false }
• Add root scripts: changeset, version:packages, release:alpha
• Set four publishable packages to 0.1.0-alpha.0: core, dom, router, testing
• Keep server private and unchanged at 0.1.0
• Update docs: Release-Readiness.md, README.md

Tests

• All 310 existing tests pass
• pnpm typecheck passes
• pnpm build passes
• pnpm lint passes
• pnpm pack:check shows all four packages at 0.1.0-alpha.0
• pnpm changeset status confirms no changesets (expected — versions set directly)
• pnpm dev:web-basic starts cleanly

Known limitations

• No automated publish workflow yet (Changesets is configured, release workflow pending)
• @intent-framework/server remains private and unpublished
• No GitHub Actions release workflow added yet

Follow-up

Add the release workflow (GitHub Action for Changesets) and npm publish automation.