feat: category assignment in create_task / update_task (v0.4.10)

Verified via FW_DEBUG=1 + systematic param-name probing that the
correct parameter is `taskCategoryId` with value = full metaId from
get_categories (e.g. taskCategory/23431854_200).  Numeric systemCategoryId
alone causes API error; full metaId is accepted and stored.

Changes:
- create_task: add optional category_id parameter → sent as taskCategoryId
- update_task: add optional category_id parameter → sent as taskCategoryId;
  guard now accepts category_id-only updates
- get_tasks: expose category_id field in returned task objects
- get_categories: update docstring (param name now known)
- SPEC.md: document verified taskCategoryId param + clarify categories[]
  vs taskCategoryId field distinction
- scripts/find_category_param.py: discovery script used to find param name

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

README.md

@@ -2,21 +2,21 @@
 
 MCP server for [Family Wall](https://www.familywall.com) -- read and manage your family's circles, lists, and tasks directly from Claude.
 
-## Features (v0.4.9)
+## Features (v0.4.10)
 
 ### Read
 
 - `get_circles` -- list all family circles
 - `get_members` -- list members of a circle (or all circles)
 - `get_lists` -- list all task lists (optionally filtered by circle)
-- `get_tasks` -- list tasks in a specific list
+- `get_tasks` -- list tasks in a specific list (includes `category_id` field)
 - `get_categories` -- list categories available for a list (locale-filtered, default: German)
 - `get_activities` -- list recent wall activities (author resolved to display name)
 
 ### Write (with confirmation prompt)
 
-- `create_task` -- create a new task in a list
-- `update_task` -- update the text/description of an existing task
+- `create_task` -- create a new task in a list (supports `category_id` for shopping lists)
+- `update_task` -- update text, description, and/or category of an existing task
 - `toggle_task` -- mark a task complete or reopen it
 - `delete_task` -- permanently delete a task
 - `like_post` -- like or unlike a wall post/activity