fix(hooks): stop cavecrew model overrides rewriting a source checkout

resolvePluginRoot returns the repo root when the hook runs from a clone, so with
any CAVECREW_*_MODEL set every SessionStart rewrote tracked agents/*.md and
dirtied the tree. Skip when the plugin root sits inside a git work tree; an
installed plugin under $CLAUDE_CONFIG_DIR still gets overrides.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SqmbqWDtNexzDfH5V3BMRz