docs: add npm/CI badges + Cost & Performance section, run tests in CI

[devplanningo]

Summary

• Add npm/CI/license/Node badges to both READMEs
• Add Cost & Performance section with empirical numbers from real DUUL usage in this repo (42 reviewer calls, gpt-5.4, prompt caching enabled)
• CI workflow now runs npm test after the build step (previously only built)

Cost data added to README

Tool	Avg tokens/call	Avg cost/call	Cache hit rate
plan_review	100,966	$0.065	79%
code_review	179,837	$0.122	79%
Combined avg	132,890	$0.088	79%

Typical task (1–3 plan rounds + 1–2 code rounds) lands around $0.30–$0.50 in reviewer cost. Includes pointer to scripts/token-report.mjs for self-measure.

Why now

• Package is published to npm (@planningo/duul@1.0.0) but README had no version/release indicators — readers couldn't tell at a glance whether the project was actively published or maintained.
• CI was building but not running tests. The npm test suite (60 tests, ~135ms) was passing locally; this PR brings CI in line.

Follow-ups noted but not in this PR

• CHANGELOG.md doesn't exist yet. .changeset/ is set up but npx changeset version hasn't been run. Should be done as part of the next versioned release.
• Quick Start block at top of README (currently first command appears only inside Installation section).
• CONTRIBUTING.md if external contributions are expected.
• Auto GitHub Release notes from changeset action.

Test plan

• npm test — 60/60 pass locally
• CI runs new test step on this PR (verify the workflow fires)
• Badges render correctly on GitHub README
• Cost table reads correctly in both en/ko READMEs

🤖 Generated with Claude Code