Add Claude Code support to nerve agent:
- nerve agent inject claude — append nerve skill block to ~/.claude/CLAUDE.md
- nerve agent remove claude — remove only the nerve block, preserve user content
- nerve agent status — show Claude Code injection status
- nerve agent update — includes Claude Code
Uses marker comments <!-- nerve-cli:start vX.Y.Z --> / <!-- nerve-cli:end -->
to safely coexist with user's existing CLAUDE.md content.
Closes#305
Ref: #289