feat(plan): start_planning MCP tool + get_task rules augmentation + instructions

src/fabledassistant/mcp/server.py

@@ -30,6 +30,12 @@ text (Why / How-to-apply) is available via get_rule(id). You may create new
 rules via create_rule when you notice a pattern worth codifying — coordinate
 with the operator on whether it belongs in an existing rulebook+topic or a
 new one.
+
+Plans are tasks with kind=plan. Begin a plan with start_planning(project_id,
+title) — that seeds a plan template and returns the project's applicable_rules.
+Maintain the plan with the normal task tools (update_task for the body,
+add_task_log for progress); its work-logs are the build record. Build plans in
+Scribe via start_planning, not in local .md files.
 """