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

install.sh

@@ -131,6 +131,17 @@ wire_skills() {
       name="$(basename "$dir")"
       target="${dir%/}"
 
+      # A skill declares its surfaces; this installer serves exactly one of
+      # them. A chat-only skill linked into Crush can never fire and competes
+      # for selection with ones that can (#29). A source with no declaration
+      # is foreign — Anthropic's official set has no reason to know this
+      # convention — so silence means "link it".
+      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
+        log "skipping $name — surfaces: $surfaces does not include the library"
+        continue
+      fi
+
       if printf '%s' "$desired" | grep -qx -- "$name"; then
         log "COLLISION: $name is offered by $target too; keeping $AGENTS_DIR/$name -> $(readlink "$AGENTS_DIR/$name" 2>/dev/null || echo '?')"
         continue