feat(headroom): add proxy lifecycle management + dashboard UI

Build on the optional Headroom Token Saver from Carmelo Campos
(PR: feat: add optional Headroom token saver). Add managed start/stop
of the local headroom proxy from the dashboard, install detection,
status probing, and a simplified Token Saver UI.

- detect headroom CLI + python>=3.10, probe proxy /health
- spawn/stop proxy as a detached, pid-tracked process
- /api/headroom/{status,start,stop} routes, gated local-only in dashboardGuard
- one-click Start/Stop Headroom modal, no manual config needed
- claude<->openai shape conversion for /v1/compress via 9router translators

Thanks to Carmelo Campos (@carmelogunsroses) for the original Headroom integration.

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/rtk/ponytail.js

@@ -0,0 +1,9 @@
+// Ponytail injector: appends the "lazy senior dev" instruction into the system
+// message of the final request body, just before dispatch to the provider executor.
+
+import { injectSystemPrompt } from "./systemInject.js";
+import { PONYTAIL_PROMPTS } from "./ponytailPrompt.js";
+
+export function injectPonytail(body, format, level) {
+  injectSystemPrompt(body, format, PONYTAIL_PROMPTS[level]);
+}