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>

workplan-spec.md

@@ -0,0 +1,71 @@
+# Workplan File Specification
+
+A workplan is a Markdown file with YAML frontmatter that describes a unit of
+work. It is the only input the `/ralph-workplan` skill needs.
+
+## File Format
+
+```markdown
+---
+id: WP-0001
+title: "Build a thing"
+status: active
+---
+
+Optional free-text description of the workplan scope and goals.
+
+## Task: Do the first thing
+
+```task
+id: T-01
+status: todo
+priority: high
+```
+
+## Task: Do the second thing
+
+```task
+id: T-02
+status: todo
+priority: medium
+```
+```
+
+## Frontmatter Fields
+
+| Field | Required | Values | Description |
+|-------|----------|--------|-------------|
+| `id` | yes | string | Unique workplan identifier |
+| `title` | yes | string | Human-readable name |
+| `status` | yes | `active` \| `done` \| `paused` | Workplan lifecycle state |
+
+## Task Blocks
+
+Each task is a fenced code block with language tag `task`, embedded anywhere in
+the document body. Fields:
+
+| Field | Required | Values | Description |
+|-------|----------|--------|-------------|
+| `id` | yes | string | Unique task identifier within the workplan |
+| `status` | yes | `todo` \| `in_progress` \| `done` | Task state |
+| `priority` | no | `high` \| `medium` \| `low` | Execution priority |
+
+## Completion Rule
+
+A workplan is **done** when:
+1. Every task block has `status: done`
+2. The frontmatter `status` field is `done`
+
+The skill updates both as work progresses — tasks individually, then the
+workplan when all tasks are complete.
+
+## Naming Convention
+
+The skill works with any filename. A common convention used in custodian-family
+projects is:
+
+```
+workplans/<PREFIX>-WP-NNNN-<slug>.md
+```
+
+but this is not required.