ci: skip unaffected platform jobs

[iarbpairs]

Summary

• add a tested changed-path classifier for platform-specific CI ownership
• keep core mandatory on every PR
• run legacy continuity, tooling, Android, Flutter, Flutter Android, and Flutter Apple only when affected
• keep the full matrix on every push to main
• preserve the required verify check while accepting intentionally skipped legs

For host-service-only PRs, this removes unrelated Android and Flutter builds from the merge critical path. Workflow-only PRs run core + tooling; the merged main push still exercises the full matrix.

Verification

• node --test scripts/ci-paths.test.mjs — 9 pass
• go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.7 .github/workflows/ci.yml
• classifier CLI smoke: host-service source selects continuity + tooling and skips all platform jobs
• this PR itself should complete with Android and Flutter jobs reported as intentionally skipped