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>

plugin/scripts/check-done.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+# check-done.sh
+# Exit 0 if the workplan is done, 1 if not.
+# Used as the pre-start guard in the ralph-workplan skill.
+#
+# Usage: check-done.sh <workplan_file>
+
+set -euo pipefail
+
+WORKPLAN_FILE="${1:?workplan_file required}"
+
+if [[ ! -f "$WORKPLAN_FILE" ]]; then
+  echo "❌ Workplan file not found: $WORKPLAN_FILE" >&2
+  exit 2
+fi
+
+# Extract status from YAML frontmatter (first occurrence between --- markers)
+STATUS=$(sed -n '/^---$/,/^---$/{ /^---$/d; p; }' "$WORKPLAN_FILE" \
+  | grep '^status:' \
+  | head -1 \
+  | sed 's/status:[[:space:]]*//' \
+  | tr -d '"'"'" )
+
+if [[ -z "$STATUS" ]]; then
+  echo "⚠️  No status field found in frontmatter of: $WORKPLAN_FILE" >&2
+  exit 1
+fi
+
+if [[ "$STATUS" == "done" ]]; then
+  exit 0
+else
+  exit 1
+fi