feat: add package download tracker skill

[webup]

What

• Add a bundled package-download-tracker skill for npm and PyPI weekly/monthly download tracking.
• Add memory-aware trend analysis that recalls prior PowerMem snapshots and saves compact observations without extending the PowerMem CLI.
• Add the package-downloads cron template with localized prompts and bundled skill install/catalog wiring for web and Tauri.

Closes #111.

Why

• Users need lightweight recurring package adoption tracking for packages such as clawmaster and powermem without repeatedly rebuilding historical context from registry APIs.
• Stored observations provide cheaper follow-up trend analysis while registry/cache data remains the source of truth for the current requested window.

How

• Implemented a Node-only skill script using npm downloads and PyPIStats APIs, local cache normalization, PyPI canonicalization, summary output, and safe PowerMem search/add command construction.
• Wired the bundled skill through backend install/sync, Tauri resources, ClawHub catalog, cron draft generation, and zh/en/ja strings.
• Added backend and web regression tests for parsing, cache behavior, memory reuse, summary consistency, bundled installation, catalog, and cron prefill behavior.

Screenshots

• Verified in OpenClaw WebUI during implementation: skills catalog, cron template prefill, cron job creation, and generated cron chat response. No screenshot files are committed.

Verification

• npm run test --workspace=@openclaw-manager/backend -- --test-name-pattern package-download-tracker
• npm test --workspace=@openclaw-manager/web -- CronPage.test.tsx
• npm run build
• npm test