feat: load API key from ~/.claude/morph/.env

Plugin hooks run as subprocesses that don't inherit the user's shell
profile, so MORPH_API_KEY from .zshrc/.bashrc is unavailable. Read the
key from ~/.claude/morph/.env (falling back to the env var), matching
the pattern used by the Discord plugin for bot tokens.

The /install and /uninstall skills now handle API key setup and
teardown alongside the CLAUDE.md compact instructions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>