test: speed legacy state migration discovery

Keep bundled legacy migration discovery on narrow setup-entry surfaces so
state-migration tests and doctor cold paths avoid unrelated channel runtime
loads. Add targeted setup feature metadata, narrow Telegram/WhatsApp legacy
contracts, and a path-only pairing SDK helper.

scripts/lib/plugin-sdk-entrypoints.json

@@ -146,6 +146,7 @@
   "channel-mention-gating",
   "channel-lifecycle",
   "channel-pairing",
+  "channel-pairing-paths",
   "channel-policy",
   "channel-send-result",
   "channel-targets",