Update: add Linux support foundation

[SI-RUI-ZHANG]

Original issue

Skill Manager release and npm distribution were macOS-only, while the app path layer already had partial Linux/XDG support. Community PR #24 from @scottzx highlighted the Linux need, but the implementation here keeps the maintainer branch focused and public-safe.

Fix

Add a small runtime platform context for macOS/Linux, expose typed settings storage paths through /api/settings, render Settings paths from backend data, add shared release target metadata, build linux-x64 and linux-arm64 release artifact names, enable npm artifact selection for Linux x64/ARM64, skip Homebrew ownership checks outside macOS, and add Linux package smoke coverage to CI/release matrices.

Changed files

.github/workflows/ci.yml
.github/workflows/release.yml
README.md
frontend/src/api/generated.ts
frontend/src/api/openapi.json
frontend/src/features/settings/api/types.ts
frontend/src/features/settings/queries.test.tsx
frontend/src/features/settings/screens/SettingsPage.tsx
frontend/src/features/settings/screens/SettingsPage.test.tsx
package.json
packaging/npm/package.json
packaging/npm/release-targets.json
packaging/npm/scripts/channel-ownership.js
packaging/npm/scripts/install.js
packaging/npm/scripts/release-targets.js
scripts/build_release.py
scripts/check_release_targets_js.cjs
scripts/release_targets.py
scripts/validate_npm_wrapper.sh
skill_manager/platform_context.py
skill_manager/paths.py
skill_manager/harness/resolution.py
skill_manager/api/routers/settings.py
skill_manager/api/schemas/init.py
skill_manager/api/schemas/settings.py
skill_manager/application/container.py
skill_manager/application/settings/presenters.py
skill_manager/application/settings/queries.py
tests/integration/test_http_api.py
tests/support/fake_home.py
tests/unit/test_backend_container.py
tests/unit/test_paths.py
tests/unit/test_platform_context.py
tests/unit/test_release_targets.py

Reviewer: @SI-RUI-ZHANG

[SI-RUI-ZHANG]

Pushed one additional commit onto this draft branch to include Linux ARM64 as part of the same Linux support PR.

The PR now targets native release artifacts for macOS ARM64/x64 and Linux x64/ARM64. I kept this as a release-target expansion only: no new runtime platform abstraction was added beyond the existing macOS/Linux platform context.

I re-ran the local validation suite before pushing. I am dispatching CI manually as well so the package-smoke matrix runs, including linux-arm64 on ubuntu-24.04-arm.