feat: CI/CD workflows, VitePress docs and Copilot instructions

[shouze]

Summary

CI/CD workflows, VitePress documentation site, Copilot instruction files, upgrade command, and community files.

Tasks

• .github/workflows/ci.yaml — format:check, lint, knip, test --coverage, cross-compile matrix
• .github/workflows/cd.yaml — build all 6 targets on tag push, GitHub Release with artifacts
• .github/workflows/docs.yml — VitePress build + GitHub Pages deploy
• .github/instructions/overview.instructions.md — project overview for Copilot
• .github/instructions/api.instructions.md
• .github/instructions/core.instructions.md
• .github/instructions/commands.instructions.md
• .github/instructions/testing.instructions.md
• docs/.vitepress/config.ts — VitePress config with sidebar
• docs/index.md — home page
• docs/guide/getting-started.md, how-it-works.md
• docs/reference/create.md, refresh.md, dispatch.md
• src/upgrade.ts — self-upgrade command using GitHub Releases
• AGENTS.md, CONTRIBUTING.md, CODE_OF_CONDUCT.md, SECURITY.md, LICENSE.md, README.md

Release process

1. Push a v*.*.* tag
2. CD workflow builds all targets and creates a GitHub Release
3. install.sh downloads the correct binary for the user's OS/arch

Acceptance criteria

• All CI checks are green on every PR
• ./dist/github-issue-ops upgrade --dry-run prints a version check
• bun run docs:build succeeds