Add add_area and update_area MCP tools

[Juxsta]

Summary

Adds two new modification operations to round out area parity with projects:

• add_area(title, tags=None) — create a new area, optionally with tags.
• update_area(id, title=None, tags=None) — rename an area and/or replace its tags. Passing tags=[] clears all tags.

Also documents the existing area_id, area_title, and list_name parameters on update_project, which are already implemented in the AppleScript bridge and MCP wrapper but missing from the update_project section of the README.

Why

get_areas exists but there was no way to create or modify areas via the MCP server — clients have to drop down to raw AppleScript or click around in the app. With this PR, an AI client can set up a fresh Things workspace end-to-end through the MCP.

Tested

• New file tests/test_area_operations.py covers:
  • add_area happy path
  • add_area with tags
  • add_area empty-title rejection
  • update_area rename
  • update_area tag replace + clear
  • update_area invalid-id error path
• End-to-end smoke test against a live Things 3 install via the @mcp.tool wrappers (add_new_area → update_existing_area → cleanup).
• Full pre-commit gauntlet matching lint.yml: py_compile, ruff check ., ruff format --check ., mypy src/, bandit -r src/ — all pass.
• Full pytest run: all 6 new tests pass. The 5 unrelated failures in tests/test_list_operations.py are pre-existing on main (format-string assertions tripping on Things 3.20's stock onboarding todos) and unchanged by this PR.

Notes

• Areas in Things only expose name and tags via AppleScript (no notes, no scheduling, no deadlines), so the surface is intentionally smaller than add_project / update_project.
• Did not add delete_area for parity with the existing API (no delete_project either) — happy to add it as a follow-up if you'd like.

Test plan

• pytest tests/test_area_operations.py passes
• Full suite has no new regressions vs main
• ruff check . clean
• ruff format --check . clean
• mypy src/ clean
• bandit -r src/ clean
• Manual end-to-end: create area, rename, retag, clear tags, delete via AppleScript