Remove opencode-rag-plugin: hangs startup without Ollama embedding backend

The RAG plugin probes a local embedding backend (Ollama on localhost:11434
by default) at init with no timeout. When Ollama isn't running, opencode
hangs after loading plugins and never starts. Dropped it from the template
and documented re-enable steps + the --pure diagnostic in the README.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

opencode.json.template

@@ -96,7 +96,6 @@
     "opencode-mem",
     "opencode-pty",
     "@tarquinen/opencode-dcp",
-    "opencode-rag-plugin",
     "opencode-command-inject",
     "opencode-notify",
     "@gotgenes/opencode-agent-identity",