Add plugin builder for one-file client install

plugin/make-plugin.{sh,ps1} stamp a user's own Freshservice API key and the
shared server token into a template and emit a personalized freshservice.plugin
(direct HTTP MCP connection — no Node/mcp-remote/config editing on clients).
Built plugins embed credentials, so *.plugin is git-ignored; the committed
template holds only placeholders. Validated with 'claude plugin validate'.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugin/template/.mcp.json

@@ -0,0 +1,13 @@
+{
+  "mcpServers": {
+    "freshservice": {
+      "type": "http",
+      "url": "__SERVER_URL__",
+      "headers": {
+        "Authorization": "Bearer __MCP_TOKEN__",
+        "X-Freshservice-Domain": "__FS_DOMAIN__",
+        "X-Freshservice-Key": "__FS_KEY__"
+      }
+    }
+  }
+}