Project-level skills intermittently become 'Unknown skill' during sessions

Description

Project-level custom skills defined in .claude/skills/SKILLNAME/SKILL.md intermittently become unavailable during a session, returning "Unknown skill" errors. Restarting Claude Code resolves the issue.

Steps to reproduce

1. Define project-level skills in .claude/skills/ with proper SKILL.md format
2. Start Claude Code — skills are listed and invocable normally
3. Continue working in the same session
4. At some point, invoking a skill (e.g. /devlog-close) returns "Unknown skill"
5. Restart Claude Code — the same skill works again

Environment

• macOS (Darwin 25.4.0, ARM64)
• Skills are in .claude/skills/SKILLNAME/SKILL.md format (project-level)
• Context window still has capacity when the error occurs (not a compaction issue)

Notes

• This is intermittent — not every session, and timing varies
• All skills in the project become unavailable at once (not just one)
• The skill files on disk are unchanged; only a restart is needed to fix
• Related but distinct from #41530 (version-specific), #21428 (cache staleness), #11632 (new skills need restart)