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>

bin/mosaic

@@ -51,6 +51,12 @@ Management:
   release-upgrade [...]  Upgrade installed Mosaic release
   project-upgrade [...]  Clean up stale SOUL.md/CLAUDE.md in a project
 
+PRD:
+  prdy <subcommand>      PRD creation and validation
+                         init    Create docs/PRD.md via Claude session
+                         update  Update existing PRD via Claude session
+                         validate Check PRD completeness (bash-only)
+
 Coordinator (r0):
   coord <subcommand>     Manual coordinator tools
                          init    Initialize a new mission
@@ -478,6 +484,44 @@ _check_resumable_session() {
   fi
 }
 
+run_prdy() {
+  check_mosaic_home
+  local subcmd="${1:-help}"
+  shift || true
+
+  local tool_dir="$MOSAIC_HOME/tools/prdy"
+
+  case "$subcmd" in
+    init)
+      exec bash "$tool_dir/prdy-init.sh" "$@"
+      ;;
+    update)
+      exec bash "$tool_dir/prdy-update.sh" "$@"
+      ;;
+    validate|check)
+      exec bash "$tool_dir/prdy-validate.sh" "$@"
+      ;;
+    help|*)
+      cat <<PRDY_USAGE
+mosaic prdy — PRD creation and validation tools
+
+Commands:
+  init [--project <path>] [--name <feature>]   Create docs/PRD.md via guided Claude session
+  update [--project <path>]                    Update existing docs/PRD.md via Claude session
+  validate [--project <path>]                  Check PRD completeness against Mosaic guide (bash-only)
+
+Examples:
+  mosaic prdy init --name "User Authentication"
+  mosaic prdy update
+  mosaic prdy validate
+
+Output location: docs/PRD.md (per Mosaic PRD guide)
+
+PRDY_USAGE
+      ;;
+  esac
+}
+
 run_bootstrap() {
   check_mosaic_home
   exec "$MOSAIC_HOME/bin/mosaic-bootstrap-repo" "$@"
@@ -550,6 +594,7 @@ case "$command" in
   sync)       run_sync "$@" ;;
   seq)        run_seq "$@" ;;
   bootstrap)  run_bootstrap "$@" ;;
+  prdy)       run_prdy "$@" ;;
   coord)      run_coord "$@" ;;
   upgrade)    run_upgrade "$@" ;;
   release-upgrade) run_release_upgrade "$@" ;;