build: publish static Linux x64 webfetch CLI

[freelw]

Summary

• add a tag-release job for x86_64-unknown-linux-musl
• reject dynamically linked Linux output and smoke the packaged binary with webfetch-cli version
• include the Linux x64 binary in publish dependencies, GitHub Release assets, SHA256SUMS, and Tencent Cloud COS uploads
• add the same static Linux build/smoke contract to pull-request CI
• document the three supported release artifacts

This aligns WebFetch CLI with the existing Browser CLI Linux x64 release path.

Validation

• cargo fmt --all -- --check
• cargo test --all-targets --locked — 27 passed
• cargo clippy --all-targets --locked -- -D warnings
• isolated Debian/musl build produced an x86-64 static PIE with no program interpreter
• packaged binary executed webfetch-cli version successfully
• Skill ZIP packaging/integrity and exact eight-file layout passed in an isolated Ubuntu environment
• shell syntax and git diff --check passed

Release boundary

No release tag or binary upload was created by this PR. The new Linux asset is published on the next v* tag through the existing release workflow.