DEFAULT_CONFIG declared "kanban" twice. Python keeps only the last
literal for a duplicate key, so the first kanban block was silently
dropped and its "auto_subscribe_on_create": True default never made it
into DEFAULT_CONFIG. The consumer in tools/kanban_tools.py masks the
miss with cfg_get(..., default=True), but the documented default was
absent from DEFAULT_CONFIG (so config templates / 'hermes config show'
omit it), and the duplicate key is a standing hazard: any future key
added to the first block would also vanish.
Merge auto_subscribe_on_create into the single canonical kanban block.
Adds a regression test asserting both default sets survive and a guard
against any duplicate top-level DEFAULT_CONFIG key.
