feat: add multi-runtime support (coord run, prdy --codex) and next-task capsule

- coord/prdy subcommands now accept --claude/--codex runtime flags
- New `mosaic coord run` generates continuation context and launches
  selected runtime, replacing manual copy/paste workflow
- Next-task capsule (.mosaic/orchestrator/next-task.json) provides
  machine-readable execution context for deterministic session launches
- Codex strict orchestrator profile added to runtime/codex/RUNTIME.md
- Orchestrator protocol updated with between-session run flow
- New smoke-test.sh for orchestration behavior verification

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

tools/prdy/_lib.sh

@@ -41,6 +41,20 @@ _require_cmd() {
   fi
 }
 
+prdy_runtime() {
+  local runtime="${MOSAIC_PRDY_RUNTIME:-claude}"
+  case "$runtime" in
+    claude|codex) echo "$runtime" ;;
+    *) echo "claude" ;;
+  esac
+}
+
+prdy_runtime_command() {
+  local runtime
+  runtime="$(prdy_runtime)"
+  echo "$runtime"
+}
+
 # ─── PRD detection ───────────────────────────────────────────────────────────
 
 # Find the PRD file in a project directory.