sausage-plugin: the crate docs are the manual for agents; remove PROPOSAL.md

Its content lives in PROTOCOL.md (the contract), DECISIONS.md (the
reasons), and the SDK docs (how to write a plugin).

sdk/imports/notes.json

@@ -0,0 +1,25 @@
+{
+  "openrpc": "1.2.6",
+  "info": {"title": "notes", "version": "0.1.0"},
+  "components": {"schemas": {
+    "AddParams": {
+      "description": "The parameters of add.",
+      "type": "object",
+      "properties": {"text": {"type": "string", "description": "The text to append."}},
+      "required": ["text"]
+    },
+    "Added": {
+      "description": "The result of add.",
+      "type": "object",
+      "properties": {"count": {"type": "integer", "minimum": 0, "description": "The total number of notes."}},
+      "required": ["count"]
+    }
+  }},
+  "methods": [{
+    "name": "add",
+    "summary": "Append a note and return the new count.",
+    "params": [{"name": "text", "required": true, "schema": {"type": "string"}}],
+    "x-sausage-params": {"$ref": "#/components/schemas/AddParams"},
+    "result": {"name": "result", "schema": {"$ref": "#/components/schemas/Added"}}
+  }]
+}