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

tests/doctor_test.sh

@@ -127,6 +127,13 @@ f=$(new_fixture missing); wire_clean "$f"
 rm -f "$f/home/.agents/skills/beta"
 assert_fails "$f" 'beta' 'a checkout skill with no link is reported'
 
+# ...but only for skills that ship to the library. A chat-only skill has no
+# business in a Crush directory, so its absence is correct, not drift (#29).
+f=$(new_fixture chatonly); wire_clean "$f"
+mkdir -p "$f/ck/skills/gamma"
+printf -- '---\nname: gamma\nsurfaces: [chat]\ndescription: g\n---\n' > "$f/ck/skills/gamma/SKILL.md"
+assert_clean "$f" 'a chat-only skill needs no library link'
+
 # --- 5. stray entry (the loose secret-hygiene.md that loaded nowhere) --------
 f=$(new_fixture stray); wire_clean "$f"
 printf 'orphan\n' > "$f/ck/skills/loose-note.md"