feat: initial ralph-workplan skill

Standalone Claude Code skill that ties a ralph loop to a workplan file.
Retires automatically when all tasks are done — no external dependencies.

- plugin/ralph-workplan.md      skill entrypoint
- plugin/scripts/check-done.sh  pre-start guard (reads workplan status)
- plugin/scripts/setup.sh       writes ralph state file with workplan-aware prompt
- install.sh                    copies plugin files to ~/.claude/plugins/
- workplan-spec.md              workplan file format reference
- README.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

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+.claude/