feat: add mosaic prdy command for PRD creation and validation

Adds `mosaic prdy {init|update|validate}` subcommand:
- init: launches yolo Claude session with PRD-focused system prompt
- update: launches session to modify existing docs/PRD.md
- validate: bash-only completeness checker (15 checks against PRD guide)

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

tools/prdy/prdy-init.sh

@@ -0,0 +1,87 @@
+#!/usr/bin/env bash
+set -euo pipefail
+#
+# prdy-init.sh — Create a new PRD via guided Claude session
+#
+# Usage:
+#   prdy-init.sh [--project <path>] [--name <feature>]
+#
+# Launches a dedicated Claude Code session in yolo mode with a specialized
+# system prompt that guides the user through PRD creation. The output is
+# written to docs/PRD.md.
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "$SCRIPT_DIR/_lib.sh"
+
+# ─── Parse arguments ─────────────────────────────────────────────────────────
+
+PROJECT="."
+NAME=""
+
+while [[ $# -gt 0 ]]; do
+  case "$1" in
+    --project) PROJECT="$2"; shift 2 ;;
+    --name)    NAME="$2"; shift 2 ;;
+    -h|--help)
+      cat <<'USAGE'
+prdy-init.sh — Create a new PRD via guided Claude session
+
+Usage: prdy-init.sh [--project <path>] [--name <feature>]
+
+Options:
+  --project <path>   Project directory (default: CWD)
+  --name <feature>   Feature or project name (optional, Claude will ask if omitted)
+
+Launches Claude Code in yolo mode with a PRD-focused system prompt.
+The agent will ask clarifying questions, then write docs/PRD.md.
+
+Examples:
+  mosaic prdy init
+  mosaic prdy init --name "User Authentication"
+  mosaic prdy init --project ~/src/my-app --name "Dashboard Redesign"
+USAGE
+      exit 0
+      ;;
+    *) echo "Unknown option: $1" >&2; exit 1 ;;
+  esac
+done
+
+# Expand tilde if passed literally (e.g., --project ~/src/foo)
+PROJECT="${PROJECT/#\~/$HOME}"
+
+# ─── Preflight checks ───────────────────────────────────────────────────────
+
+_require_cmd "claude"
+
+# Check for existing PRD
+EXISTING="$(find_prd "$PROJECT")"
+if [[ -n "$EXISTING" ]]; then
+  fail "PRD already exists: $EXISTING"
+  echo -e "  Use ${C_CYAN}mosaic prdy update${C_RESET} to modify the existing PRD."
+  exit 1
+fi
+
+# Ensure docs/ directory exists
+mkdir -p "$PROJECT/docs"
+
+# ─── Build system prompt ─────────────────────────────────────────────────────
+
+step "Launching PRD creation session"
+
+SYSTEM_PROMPT="$(build_prdy_system_prompt "init")"
+
+# Build kickoff message
+if [[ -n "$NAME" ]]; then
+  KICKOFF="Create docs/PRD.md for the feature: ${NAME}. Read the project context first, then ask clarifying questions before writing the PRD."
+else
+  KICKOFF="Create docs/PRD.md for this project. Read the project context first, then ask the user what they want to build. Ask clarifying questions before writing the PRD."
+fi
+
+# ─── Launch Claude ───────────────────────────────────────────────────────────
+
+info "Output target: $PROJECT/$PRD_CANONICAL"
+info "Mode: PRD Creation (yolo)"
+echo ""
+
+cd "$PROJECT"
+exec claude --dangerously-skip-permissions --append-system-prompt "$SYSTEM_PROMPT" "$KICKOFF"