Rename packages/core to packages/react-simplikit, absorb the mobile package, add subpath imports

[mnxmnz]

Evolve react-simplikit as a general-purpose utility library and consolidate the mobile package back into the main one.

As-is

• The main package lives in packages/core, which doesn't match its published name react-simplikit.
• Mobile web hooks/utils are published separately as @react-simplikit/mobile (packages/mobile).
• Consumers need two installs and two import roots for one library.

To-be

• packages/core → packages/react-simplikit, so the directory matches the published package.
• Move the hooks/utils from packages/mobile into packages/react-simplikit.
• Expose them via a subpath import (e.g. react-simplikit/mobile); the exact import shape (root re-export vs subpath-only) is decided in the PR.

Why

One general-purpose package is the direction we committed to; the core/mobile split (#329) optimized for a mobile-focused roadmap we're no longer pursuing.

Must verify

• CI path filters, coverage thresholds, and docs-site rewrites still point at the renamed directory (they broke once before: CI path filters still target src/** after the monorepo migration #390).
• A migration/deprecation path for the published @react-simplikit/mobile npm package.