feat(plugins): add document CRUD methods to Plugin SDK

Wire issue document list/get/upsert/delete operations through the
JSON-RPC protocol so plugins can manage issue documents with the same
capabilities available via the REST API.

Fixes #940

packages/shared/src/constants.ts

@@ -315,6 +315,7 @@ export const PLUGIN_CAPABILITIES = [
   "project.workspaces.read",
   "issues.read",
   "issue.comments.read",
+  "issue.documents.read",
   "agents.read",
   "goals.read",
   "goals.create",
@@ -325,6 +326,7 @@ export const PLUGIN_CAPABILITIES = [
   "issues.create",
   "issues.update",
   "issue.comments.create",
+  "issue.documents.write",
   "agents.pause",
   "agents.resume",
   "agents.invoke",