docs: add opencode-scratchpad to plugins (#477)

Co-authored-by: adikpb <[email protected]>

data/plugins/opencode-scratchpad.yaml

@@ -0,0 +1,8 @@
+name: OpenCode Scratchpad
+repo: https://github.com/adikpb/opencode-scratchpad
+tagline: A private per-project scratchpad you can capture and edit without leaving the TUI
+description: One plain-Markdown scratchpad per project, keyed by absolute path. Append a line with `/scratch <text>` or open a modal editor with `/scratch`; the session sidebar shows a collapsible, read-only preview that refreshes on save. Notes live under `$OPENCODE_DATA_DIR/scratchpad` and are never written into your repo.
+tags:
+  - scratchpad
+  - notes
+  - tui