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>

.gitignore

@@ -6,3 +6,7 @@ __pycache__/
 
 # MCP server runtime secrets
 mcp-server/.env
+
+# Personalized plugin builds contain API keys — never commit
+*.plugin
+!freshservice-skill.zip