#631 Implement tools.scriptplan - Advanced Scheduling Tool

## Overview Implement tools.scriptplan as a native Highway tool for advanced scheduling using the ScriptPlan library (TaskJuggler-compatible). This provides Layer 3 scheduling capabilities beyond the built-in durable_cron. ## Architecture Decisions 1. Tool, NOT App - Lives in engine/tools/scriptplan.py (native primitive) 2. No custom DB tables - All state in workflow variables (ctx.get/set_variable) 3. Uses existing patterns - State reset pattern from durable_cron.py 4. Pre-computed schedules - Compile once, cache execution times list 5. Interruptible waits - wait_for_event() with timeout for schedule updates ## Tool API - compile_schedule(definition, task_id, count=100) -> list[str] - get_next_execution(definition, task_id, after=None) -> str|None - scriptplan_scheduler(ctx, definition, task_id, target_task, ...) -> dict ## State Variables (via ctx.get/set_variable) - execution_times: list[str] - Pre-computed ISO timestamps - last_run: str - Last execution ISO timestamp - schedule_hash: str - Hash for change detection - scheduler_id: str - Unique ID for event-based wake ## Implementation Steps 1. Add scriptplan dependency to pyproject.toml 2. Create engine/tools/scriptplan.py with core functions 3. Implement scriptplan_scheduler with state reset pattern 4. Register tool in engine/tools/__init__.py 5. Add DSL template example 6. Write unit tests 7. Integration test with durable scheduler 8. Update RAG documentation ## Dependencies - ScriptPlan library from ../rodmena-resource-management - Existing: durable_context.py, absurd_client.py