``discord_skill_commands_by_category`` was lagging the flat
``discord_skill_commands`` collector on two counts. Both were actively
dropping skills from Discord's ``/skill`` autocomplete dropdown.
1. External-dir skills were filtered out. #18741 widened the flat
collector to accept ``SKILLS_DIR + skills.external_dirs`` but left
this sibling collector — the one ``_register_skill_group`` actually
uses on Discord — still matching ``SKILLS_DIR`` only. External
skills were visible in ``hermes skills list`` and the agent's
``/skill-name`` dispatch but silently absent from Discord's
``/skill`` picker. Widen the accepted roots to match, and derive
categories from whichever root the skill lives under so
``<ext>/mlops/foo/SKILL.md`` still lands in the ``mlops`` group.
2. 25-group × 25-subcommand caps were still applied. PR #11580
refactored ``/skill`` to a flat autocomplete (whose options Discord
fetches dynamically — no per-command payload concern) and its
docstring promises "no hidden skills." The collector kept the old
nested-layout caps anyway, silently dropping anything past the 25th
alphabetical category. On installs with 29 category dirs today (real
example: tail categories ``social-media``, ``software-development``,
``yuanbao`` going missing) this was biting immediately. Remove the
caps; ``hidden`` now reports only 32-char name-clamp collisions
against reserved names.
Tests: guard both behaviors. ``test_no_legacy_25x25_cap`` builds 30
categories × 30 skills each and asserts all 900 are returned.
``test_external_dirs_skills_included`` monkeypatches
``get_external_skills_dirs`` and asserts an external-dir skill makes
it into the result grouped under its own top-level directory.
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