feat: rewrite SKILL.md, enhance README install section, add CI workflow

[Copilot]

Overhauls the skill definition, installation documentation, and CI/CD setup. The previous SKILL.md was sparse, README install instructions were incomplete (no PATH guidance, no Windows automation), and there was no CI workflow for validating PRs.

skill/SKILL.md

• Enriched front-matter: added tags, switched install kind to release, expanded env-var descriptions
• Added full internals walkthrough (upload → queue → SSE stream → ZIP extract → format)
• Four install paths with copy-paste commands: one-liner curl (macOS Universal / Linux x86_64 / ARM64), Windows PowerShell with automatic PATH registration, cargo build --release, cargo install
• PATH tips for rootless installs ($HOME/.local/bin)
• Complete option reference with per-flag explanations
• Grouped examples: basic, file output, LLM pipes, advanced (proxy, VLM backend, PPTX)
• Output format samples (Markdown with LaTeX, JSON schema, plain text)
• Limits/performance table; backend comparison table with use-case guidance

README.md

• Added CI / Release / version / MIT badges
• Replaced vague "download from Releases" with curl | tar one-liners placing binary directly in /usr/local/bin/
• Windows PowerShell script: versioned download → extract → %LOCALAPPDATA%\Programs\mineru → permanent user PATH update
• cargo install and source-build-to-PATH options
• Expanded binary table with architecture column

.github/workflows/ci.yml (new)

• Triggers on push/PR to main/master
• Three jobs: rustfmt check, clippy (-D warnings), build + test (matrix: ubuntu / macos / windows)
• Cargo registry cache; permissions: contents: read

.github/workflows/release.yml

• skill/SKILL.md now bundled in every release archive (.tar.gz and .zip)