feat: add native installer and release assets

[DizzyFrog]

Summary

Add a native-style one-command installer for macOS, Linux, and WSL, backed by GitHub Release assets.

What changed

• Add install.sh with Bun detection/official installation, Release download and SHA-256 verification, versioned installation with rollback-safe activation, active-pair protection, --version, --yes, --dry-run, prerequisite checks, and best-effort plugin registration.
• Extend the existing publish.yml to upload install.sh, agentbridge.tgz, and its checksum.
• Skip npm publish/verification for forks while retaining it for raysonmeng/agent-bridge.
• Document native installation in both English and Chinese README files.
• Add fixture-based installer tests.

Validation

• bun run typecheck — passed
• bun test src/unit-test/native-installer.test.ts — 4 passed
• bash -n install.sh — passed
• Linux Ubuntu 24.04 Codespace smoke test — passed
• Fork Release asset workflow — passed

bun test src/ has four pre-existing Claude/kill integration-test timeouts; the same failures reproduce on a clean upstream/master baseline. No runtime TypeScript files were changed by this PR.

The installer does not run abg init; project initialization remains explicit.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[DizzyFrog]

I have read the CLA Document and I hereby sign the CLA

[DizzyFrog]

recheck

[DizzyFrog]

recheck

[DizzyFrog]

@raysonmeng The CLA workflow is currently awaiting your approval because this PR comes from a
first-time contributor fork. Could a maintainer approve and run the workflow