ci: replace 10 workflows with pr-build.yml + release.yml

Two new workflows replace the entire per-SDK release fleet:

- .github/workflows/pr-build.yml: PR-triggered, path-filtered. A change
  in a single backend's C++ does not rebuild every other backend, and a
  change in a single client SDK does not rebuild any natives. Uses
  dorny/paths-filter@v3 + per-job `if:` gates per the trigger map in
  thoughts/shared/plans/artifact-build-system.md §6.2. Native build
  outputs go to workflow artifacts (30-day retention).

- .github/workflows/release.yml: Tag-triggered (v*). Full matrix builds
  all native artifacts (commons + llamacpp + onnx across iOS/macOS/
  Android × 4 ABIs / Linux / Windows / Web) plus Web npm tarballs and
  Kotlin AAR/JAR. Computes SHA-256 next to each. Attaches everything to
  one GitHub Release. No registry publishing (no npm, Maven Central,
  pub.dev, GitHub Packages).

Workflows deleted (10):
- build-all-test.yml          → replaced by pr-build.yml
- commons-release.yml         → folded into release.yml
- backends-release.yml        → folded into release.yml
- swift-sdk-release.yml       → folded into release.yml
- kotlin-sdk-release.yml      → folded into release.yml
- web-sdk-release.yml         → folded into release.yml
- flutter-sdk-release.yml     → folded into release.yml
- react-native-sdk-release.yml→ folded into release.yml
- release-all.yml             → replaced by release.yml
- publish-maven-central.yml   → no Maven publishing

Scripts deleted:
- sdk/runanywhere-web/scripts/publish.sh — no npm publishing

Final workflow set: codeql.yml, secret-scan.yml, pr-build.yml,
release.yml.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sanchitmonga22