skill install copies companion directories verbatim, with no exclusion mechanism #103

A skill directory often carries assets that belong in the registry but not in a platform's skill directory — evaluation corpora, fixtures, development scratch. skern installs the whole tree.

Reproduced on skern 0.3.1: a template with eval/s1.md alongside SKILL.md installs as:

.claude/skills/demo.skill/SKILL.md
.claude/skills/demo.skill/eval/s1.md

Everything under the skill directory is loaded or at least indexed by the host agent, so a large eval corpus is dead weight in the agent's context budget on every project.

Requested behavior: an exclusion mechanism at install time — a repeatable --exclude <glob> flag, a convention (a leading-dot or _-prefixed directory), or an install.exclude list in the skill's frontmatter. Any of the three solves it; the frontmatter form is the most portable since the skill author knows which of its own files are runtime assets.

Definition of Done

• Named directories/globs under a skill can be excluded from skill install while staying in the registry
• skill diff accounts for intentionally-excluded paths rather than reporting them as drift