feat(dev): add local harness installer

[LenoSeibert]

Summary

• add an idempotent local installer for Codex, Claude Code, Gemini CLI, and GitHub Copilot CLI
• test modified and untracked source without pushing to GitHub
• isolate local marketplaces under semver-plugin-local to avoid collisions with published installs
• add Codex cachebuster versions and a --dry-run mode

Usage

./scripts/install-local.sh all
./scripts/install-local.sh codex
./scripts/install-local.sh all --dry-run

Validation

• bash -n scripts/install-local.sh
• isolated Codex installation from source: local
• node scripts/check-version.js
• Codex plugin manifest validation
• git diff --check