feat: add native Rust WorkBuddy WebFetch skill

[freelw]

Summary

• implement the Python webfetch-cli agent-facing contract as a native Rust SDK and CLI
• support extract, dump-dom, Markdown/text/compact JSON/full JSON output, quality warnings, capabilities, doctor, Skill install, and PKCE authentication
• package a concise lexmount-webfetch WorkBuddy Skill with fixed-version SHA-verified installers
• publish exactly two raw binaries (macOS ARM64 and Windows x64) plus a direct-upload SkillHub ZIP and SHA256SUMS

Compatibility

• POST /v1/extract and POST /v1/dom/dump request bodies match Python behavior
• debug fields require --format json-full
• default output remains agent-readable Markdown
• environment and credential-file names match the Python CLI

Validation

• cargo test --all-targets --locked (7 passed)
• cargo clippy --all-targets --locked -- -D warnings
• cargo fmt --all
• Skill Creator quick_validate.py
• deterministic ZIP generation and archive integrity/root-layout assertions
• workflow YAML parsing and exactly-two-target release contract assertion (no Linux or macOS Intel)
• CLI help/capabilities/auth-missing/debug-guard smoke tests