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-update.sh

@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+set -euo pipefail
+#
+# prdy-update.sh — Update an existing PRD via guided Claude session
+#
+# Usage:
+#   prdy-update.sh [--project <path>]
+#
+# Launches a dedicated Claude Code session in yolo mode with a specialized
+# system prompt that reads the existing PRD and guides targeted modifications.
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "$SCRIPT_DIR/_lib.sh"
+
+# ─── Parse arguments ─────────────────────────────────────────────────────────
+
+PROJECT="."
+
+while [[ $# -gt 0 ]]; do
+  case "$1" in
+    --project) PROJECT="$2"; shift 2 ;;
+    -h|--help)
+      cat <<'USAGE'
+prdy-update.sh — Update an existing PRD via guided Claude session
+
+Usage: prdy-update.sh [--project <path>]
+
+Options:
+  --project <path>   Project directory (default: CWD)
+
+Launches Claude Code in yolo mode with a PRD-update system prompt.
+The agent will read the existing docs/PRD.md, summarize its state,
+and ask what changes are needed.
+
+Examples:
+  mosaic prdy update
+  mosaic prdy update --project ~/src/my-app
+USAGE
+      exit 0
+      ;;
+    *) echo "Unknown option: $1" >&2; exit 1 ;;
+  esac
+done
+
+# Expand tilde if passed literally (e.g., --project ~/src/foo)
+PROJECT="${PROJECT/#\~/$HOME}"
+
+# ─── Preflight checks ───────────────────────────────────────────────────────
+
+_require_cmd "claude"
+
+# Require existing PRD
+EXISTING="$(find_prd "$PROJECT")"
+if [[ -z "$EXISTING" ]]; then
+  fail "No PRD found in $PROJECT/docs/"
+  echo -e "  Run ${C_CYAN}mosaic prdy init${C_RESET} to create one first."
+  exit 1
+fi
+
+# ─── Build system prompt ─────────────────────────────────────────────────────
+
+step "Launching PRD update session"
+
+SYSTEM_PROMPT="$(build_prdy_system_prompt "update")"
+
+KICKOFF="Read the existing PRD at ${EXISTING}, summarize its current state, then ask what changes or additions are needed."
+
+# ─── Launch Claude ───────────────────────────────────────────────────────────
+
+info "Updating: $EXISTING"
+info "Mode: PRD Update (yolo)"
+echo ""
+
+cd "$PROJECT"
+exec claude --dangerously-skip-permissions --append-system-prompt "$SYSTEM_PROMPT" "$KICKOFF"