Modernize dependencies, fix bugs, clean up tests, add CI

[dkzlv]

Summary

Dependencies & build

• Update all dependencies to current versions: vite 5->6, vitest 1->3, react 18->19, nanostores 0.10->1.2, typescript 5.4->5.8, happy-dom 14->17, @testing-library/react 14->16, @rollup/plugin-replace 5->6, @nanostores/react 0.7->1.1, lefthook 1->2
• Drop EOL Node 14/16 from the engines field (now >=20.0.0)
• Add GitHub Actions CI -- full build+test suite on Node latest, plus a matrix job for Node 20 and 22

Bug fixes

• Fix duplicate setKey bug in createMutatorStore -- the error handler called both safeKeySet("error", ...) and store.setKey("error", ...), bypassing the listener-count guard on unsubscribed stores
• Fix React Native module resolution (Cannot find module '@nanostores/query/react-native' #67) -- add import and default conditions to ./react-native subpath export, add default fallback to main export

Test improvements

• Remove all console.log debug logging from factory.ts -- these were stripped in production builds but flooded test output with hundreds of lines of noise
• Rewrite advance() test helper to use vi.advanceTimersByTimeAsync() instead of the old hack (self-described as "I don't know what I'm doing") that manually flushed microtasks with repeated new Promise(r => r()) calls
• Simplify test patterns -- removed dozens of redundant multi-advance flush sequences, removed empty placeholder test
• Fix React 19 act() warning in the conditional render integration test

Documentation

• Fix README typos: "Automaticallty" -> "Automatically", "createMutationStore" -> "createMutatorStore", "accept and argument" -> "accept an argument", extra double-spaces, outdated size claim
• Document all store states (Document all store states #75) -- added tables for both fetcher and mutator stores covering every possible {loading, data, error} combination and key behaviors
• Fix misleading usage example (Update docs: loading state is false on first render #69) -- noted that initial state before subscription is {loading: false}, updated all code examples to check data first and handle error explicitly
• Add Agents.md documenting architecture, source structure, build system, and conventions
• Add REFRESH_PLAN.md with prioritized improvement roadmap

Addresses

• Cannot find module '@nanostores/query/react-native' #67 (Cannot find module @nanostores/query/react-native)
• Update docs: loading state is false on first render #69 (Update docs: loading state is false on first render)
• Document all store states #75 (Document all store states)

Test plan

• All 55 tests pass locally (4 test files, 0 type errors)
• Build succeeds (pnpm build)
• Size-limit check passes (1.91 KB, under 1.92 KB budget)
• CI workflow passes on GitHub Actions

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