docs: improve core package docs

[aose-yuu]

Purpose

Make the core package metadata and README easier to scan for first-time readers.

Summary

• Clarifies the package description and keywords around reactive primitives, deep signals, scopes, and molecule lifecycles.
• Fixes README code examples to rely on props parameter typing so molecule return types stay inferred.
• Points the README mascot image at a GitHub raw URL so it still renders from the npm package README.
• Tightens wording around plain helpers, tracked molecule cleanup, and examples that use @sigrea/use.

Verification

• pnpm -s cicheck
• Sub-agent review: remaining must-fix findings none