build_skills_system_prompt() was using the skill directory name (skill_name)
when appending to skills_by_category in all three code paths (snapshot cache,
cold filesystem scan, external dirs). This meant any skill whose directory name
differed from its frontmatter `name` field would appear under the wrong name in
the system prompt, causing LLM routing failures.
The snapshot entry already stores both skill_name (dir) and frontmatter_name
(declared); switch the three tuple appends to use frontmatter_name. Also fix
the external-dir dedup set (seen_skill_names) to track frontmatter names for
consistency with the local-skill tuples now stored under frontmatter_name.
Fixes#11777
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
d465fc5869