ci: add GitHub Actions verify pipeline

[btspoony]

Summary

User request (2026-08-14): add CI configuration. New .github/workflows/ci.yml — verify pipeline for PRs + main pushes:

• Triggers: pull_request, push: main, workflow_dispatch
• Concurrency group per workflow+ref (stale runs cancelled)
• Job build-and-test (ubuntu-latest, permissions: contents: read, timeout-minutes: 20):
  checkout@v4 → pnpm/action-setup@v4 (11.21.0) → setup-node@v4 (22.23.2, cache pnpm) → pnpm install --frozen-lockfile → pnpm test → pnpm build (tsc + tsdown + build-client + verify-dist)

Zero-publish, zero secrets (verified: @deepseek-ai/* peers resolve from the public registry, no .npmrc).

Verification

• actionlint clean + YAML parse (implementer evidence)
• QC tri 3× Approve (0 Critical/Warning) → fix wave e239eff → targeted re-review 3× Approve
• QA (mandatory): real PR run green (this run)
• Open residual R-001 (defer, low): double-build idempotent redundancy (prepare during install + explicit build) — tracked with Durable Roadmap in status.json