A skill's SKILL.md frontmatter may declare session-start (output into
<skill_session_context>, at session start, /new, resume and after compaction),
turn-end (after every run, in the background; output or a failure reaches the
next run as a hidden notice, repeats suppressed until a clean run) and
protected-paths (write/edit refused inside them). Hooks run from the skill
directory with VOLCANIC_WORKSPACE, VOLCANIC_SKILL_DIR and the chat's TZ; waits
are bounded by each hook's timeout and failures never break a run. /skills
shows declared hooks and broken fields.
A skill directory may hold INSTRUCTIONS.md, with or without SKILL.md. Its body
(frontmatter cut) goes into every system prompt in <skill_instructions>, re-read
on every run: 16 KB per file, 64 KB in total, truncation marked with the file's
path. /skills marks skills with instructions and lists instruction-only dirs.
Skills live in <cwd>/.claude/skills/<name>/SKILL.md (frontmatter name + description).
SkillCatalog re-reads them on every run and the prompt lists name, description and
path; the model reads the full SKILL.md with the read tool when a task matches.
