Make scenario_directive fully generic, remove project-specific knowledge

Three changes:

1. Remove hardcoded _NON_COMMAND_TAGS, _TAG_LABELS, _TAG_ORDER constants.
   The directive no longer knows anything about dfetch commands.

2. Sort appendix sections alphabetically by tag.  The previous fixed
   ordering (update, check, add, …) was dfetch-specific.

3. Expose scenario_non_command_tags as a conf.py config value (default []).
   Any tag in that list is skipped when choosing the group tag for a
   feature file. dfetch's conf.py now sets:

       scenario_non_command_tags = ["remote-svn"]

   Section titles are derived from the tag itself (title-cased, hyphens
   replaced by spaces), so no label map is needed.

https://claude.ai/code/session_01BvyikyxX9c3sDXev8HdP4f

Author: claude