fix(ci): predict release impact and align Nightly build contracts

[wgqqqqq]

Summary

Use risk-based CI for pull requests while keeping full install packages in Nightly.

• Ordinary changes run the smallest relevant frontend, Rust, or CLI checks.
• Linux CLI and Relay release paths selectively run the real Linux producer.
• Pull-request CI never runs Desktop or Installer packaging and does not create a skipped Desktop check.
• PR producers cannot upload packages or write shared build caches.
• Scheduled and manual Nightly remain the full Desktop, Installer, Linux CLI, and Relay safety net.

Selection

Change	PR behavior
Docs or native mobile only	Skip expensive validation
Web UI or Mobile Web	Frontend checks
Rust or CLI code	Focused Rust and CLI matrix
Linux or Relay release paths	Add Linux x64 and arm64 producers
CI, release, or uncertain shared paths	Fail closed to full routine checks plus Linux producers

Mixed changes take the union of all applicable checks.

Scope

CI workflows, impact classification, and their contract tests only. No Rust product code, Installer dependency changes, package manifests, or lockfiles.

Verification

• pnpm run check:github-config: 19 passed, 1 PowerShell-only case skipped locally
• Build-impact classifier: 6 passed
• Current PR run: https://github.com/GCWing/BitFun/actions/runs/32835079476
• Live job graph contains only the two Linux/Relay producer jobs; no Desktop package job or skipped Desktop check

Runtime remote scenarios are unchanged; this PR only changes validation and release orchestration.