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>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "mcp-familywall"
-version = "0.4.9"
+version = "0.4.10"
 description = "MCP server for Family Wall — read your family's lists and tasks via Claude"
 readme = "README.md"
 requires-python = ">=3.12"