feat: add isolated GitHub two-instance E2E workflow

[iHildy]

Summary

• move environment bootstrap and E2E harness scripts to top-level scripts/ so they are not Codex-specific
• add hardened two-instance GitHub E2E harness with per-run sandbox isolation, dynamic ports, plugin packaging, and strict cleanup guards
• update sandbox skill/docs/env wiring and README usage examples for setup + E2E workflow
• add process-group/port-based teardown hardening for opencode serve cleanup
• scope vitest to src/**/*.test.ts so .memory/ artifacts do not pollute repo test runs

Validation

• python3 -m py_compile scripts/e2e/github_two_instance.py
• bun run check
• bun test
• bun run build
• python3 scripts/e2e/github_two_instance.py --keep-artifacts
• parallel run test with two concurrent harness invocations from separate worktrees

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive sandbox-first workflow and an automated end-to-end (E2E) test harness for the opencode-sync project. Key additions include a two-instance GitHub integration test script, environment setup scripts, and updated documentation for the Codex environment. The PR also refines the prompt templates for the sync-init and sync-link commands. Review feedback suggests improving the safety of command-line string construction using the shlex module, ensuring pgrep is included in preflight dependency checks, and removing unnecessary backslashes from escaped quotes in markdown templates to prevent literal interpretation by LLMs.