Remove periodic-note support

Periodic notes are end-of-life in the 5.x line: the /periodic/ REST
endpoints, the periodic_note_get_path MCP tool, the native
periodic-notes settings backend and UI, and all associated types,
error codes, docs, and tests are removed. A separate plugin providing
periodic-note routes via this plugin's API extension interface is
planned as the replacement.

This clears the way for lowering the minimum Obsidian version: the
periodic-notes settings UI was one of the two consumers of the 1.13
declarative settings framework, and its settings surface no longer
needs porting to the pre-1.13 imperative settings API.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>