[Build] Add Linux packaging (AppImage + deb)

[samzong]

Summary

Add Linux desktop packaging (AppImage + deb, x64) to the CI/CD pipeline. Closes #167.

Type of change

• [Build] CI, packaging, or tooling change

Why is this needed?

ClawWork ships macOS and Windows builds but has no Linux support. AppImage + deb covers the majority of Linux desktop users with minimal maintenance overhead.

What changed?

• Added linux target section to electron-builder.yml (AppImage + deb, x64)
• Added build:linux script to desktop package.json
• Added ubuntu-latest matrix entry to release.yml, dev-release.yml, and pr-check.yml
• Added latest-linux.yml, .AppImage, and .deb asset verification to verify-release job
• Added Linux artifact globs (including latest-linux.yml) to dev-release.yml upload step

Architecture impact

• Owning layer: none (CI/packaging config only)
• Cross-layer impact: none
• Invariants touched from docs/architecture-invariants.md: none
• Why those invariants remain protected: no application code changed

Linked issues

Closes #167

Validation

• pnpm lint
• pnpm test
• pnpm check:ui-contract
• Manual smoke test
• Not run — Linux build will be validated by CI on this PR

Commands, screenshots, or notes:

pnpm check passed locally (lint + format + architecture + UI contract + i18n + typecheck + test).
Full Linux packaging will be validated by the new pr-check matrix entry on ubuntu-latest.

Screenshots or recordings

N/A — no UI changes.

Release note

• No user-facing change. Release note is NONE.
• User-facing change. Release note is included below.

Added Linux desktop builds: AppImage (universal, portable) and deb (Ubuntu/Debian) packages for x64.

Checklist

• The PR title uses at least one approved prefix: [Feat], [Fix], [UI], [Docs], [Refactor], [Build], or [Chore]
• The summary explains both what changed and why
• Validation reflects the commands actually run for this PR
• Architecture impact is described and references any touched invariants
• Cross-layer changes are explicitly justified
• The release note block is accurate

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