Claude Code Plugin: Bootstrap Python Deps in Hook Script

Bootstrap Python deps from a Claude Code plugin's hook script so `/plugin install` is one-click on systems without those deps already installed. Use when: (1) building a CC plugin whose hook (PreToolUse, PostToolUse, etc.) runs a Python script that imports third-party packages (tree-sitter, openai, requests, ...), (2) `/plugin install` succeeds but the hook silently no-ops on fresh systems with ImportError, (3) users hit `externally-managed-environment` (PEP 668) errors on recent Debian / Homebrew Python. Covers marker-file caching, content-hash invalidation on requirements bumps, the `--user` then `--break-system-packages` fallback chain, and the never-break-session inva…