docs(mcp): add conceptual primer (what each entity is for) to MCP instructions

src/fabledassistant/mcp/server.py

@@ -11,6 +11,23 @@ store. You (Claude) are the assistant.
 
 Hierarchy: Project -> Milestone -> Task/Note.
 
+What each part is for, and when to reach for it:
+- Project: the top-level container for a body of work.
+- Milestone: groups related tasks within a project toward a goal (status
+  active/done). Use one when a chunk of work needs its own arc.
+- Task: a unit of actionable work with a lifecycle (status
+  todo/in_progress/done/cancelled, optional priority). A task is a note with a
+  status — reach for one when there is something to DO. Record progress over
+  time with work-logs (add_task_log) rather than rewriting the body.
+- Plan: a task with kind=plan — HOW you'll execute a chunk of work. The body
+  holds the design + step checklist; work-logs record progress. Start one with
+  start_planning when beginning non-trivial work, before writing code.
+- Note: durable free-form knowledge — reference material, decisions, dev-logs.
+  No lifecycle, not actionable. Reach for one to CAPTURE something worth keeping.
+- Typed entities (person/place/list): structured records about people, places,
+  and checklists.
+
+Mechanics:
 - Notes and Tasks share a model; tasks are notes with is_task=True.
 - Use the *_note tools for notes, the *_task tools for tasks. Don't mix them.
 - Typed entities (person, place, list) are notes with a non-default note_type