feat(skills): declare which surfaces each skill ships to

The repo distributes three ways — the Claude Code plugin, the symlinked
library for Crush/Pi/OpenCode, and claude.ai, which has no fetch path at all —
and treated every skill as belonging to all of them. claude.ai is not a fourth
host to mirror: gitea-ci, playwright and web-shot have no shell and no cluster
there, so a flat mirror installs skills that can never fire and degrades
selection for the ones that can.

Every SKILL.md now carries `surfaces:`, defaulting to [claude-code, library];
dream, own-the-task and grill-me add `chat`, matching what is actually
installed on claude.ai today. Claude Code accepts the extra key —
`claude plugin validate` passes and the skills still load.

bin/check-surfaces validates values, not just presence, because the failure
mode here is silent: a wrong surface removes a skill from a harness rather than
erroring. install.sh links only skills whose surfaces include `library`, and a
source that declares nothing at all is treated as foreign and linked as-is —
Anthropic's official set has no reason to know this convention. doctor's
missing-link check skips non-library skills for the same reason.

Refs #29
Bump-Type: minor

bin/doctor

@@ -153,6 +153,13 @@ elif [ -d "$AGENTS_DIR" ]; then
   for dir in "$SKILLS_DIR"/*/; do
     [ -f "$dir/SKILL.md" ] || continue
     name=$(basename "$dir")
+    # Only skills that ship to the library belong here. A chat-only skill
+    # missing from a Crush directory is correct, not drift (#29).
+    surfaces=$(sed -n 's/^surfaces:[[:space:]]*//p' "$dir/SKILL.md" | head -1)
+    if [ -n "$surfaces" ] \
+       && ! printf '%s' "$surfaces" | tr -d '[]' | tr ',' ' ' | grep -qw library; then
+      continue
+    fi
     [ -L "$AGENTS_DIR/$name" ] \
       || add FAIL missing "$name is in the checkout but has no link in $AGENTS_DIR — re-run install.sh"
   done