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

Taskfile.yml

@@ -99,7 +99,12 @@ tasks:
         #    tests/check_index_test.sh rather than living untested here.
         sh '{{.TASKFILE_DIR}}/bin/check-index' '{{.TASKFILE_DIR}}' || fail=1
 
-        # 4. Plugin manifest version must match the latest tag, or installers
+        # 4. Every skill says which surfaces it ships to, and names only
+        #    surfaces that exist — a wrong value silently drops a skill from a
+        #    harness instead of erroring.
+        sh '{{.TASKFILE_DIR}}/bin/check-surfaces' '{{.TASKFILE_DIR}}' || fail=1
+
+        # 5. Plugin manifest version must match the latest tag, or installers
         #    never see the update (Claude Code gates updates on this field).
         if [ -f '{{.PLUGIN_MANIFEST}}' ]; then
           manifest_v=$(sed -n 's/.*"version"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p' '{{.PLUGIN_MANIFEST}}' | head -1)