Get project planning guidance

Returns the planning skill guidance used to draft a valid project plan for submit_plan, in three parts: plan (default: the planning contract, the plan shape and the sequence rules), search (writing the search prompts) and screening (writing the screening criteria). Read all three before drafting. Email templates are NOT included here — fetch those via get_email_template. Part of the project-planning flow — the plan_a_project prompt orchestrates create_project → get_planner_skills → get_email_template → submit_plan; start there when the user wants to run outreach.