refactor(open-sse): DRY SSE primitives into utils/sseConstants.js

Gom SSE_DONE + SSE_HEADERS + SSE_HEADERS_NO_BUFFER vào 1 file không
phụ thuộc (tránh kéo usageDb vào executor). Áp cho kiro, cursor, qoder,
github, commandcode, perplexity-web, grok-web. Byte-for-byte verified
(headers/DONE giữ nguyên; biến thể no-buffer dùng const riêng).

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

open-sse/utils/sseConstants.js

@@ -0,0 +1,15 @@
+// Shared SSE primitives (no imports → safe for executors + stream.js)
+export const SSE_DONE = "data: [DONE]\n\n";
+
+export const SSE_HEADERS = {
+  "Content-Type": "text/event-stream",
+  "Cache-Control": "no-cache",
+  "Connection": "keep-alive"
+};
+
+// Variant for web-cookie executors behind nginx (disable proxy buffering)
+export const SSE_HEADERS_NO_BUFFER = {
+  "Content-Type": "text/event-stream",
+  "Cache-Control": "no-cache",
+  "X-Accel-Buffering": "no"
+};