fix: pin @opencode-ai/sdk to 1.2.6 to fix startup crash

Versions 1.2.7–1.2.9 have a broken npm publish where compiled files
land in dist/src/ but the package exports map references dist/v2/,
causing ERR_MODULE_NOT_FOUND on startup. Pin to 1.2.6 which has the
correct layout.

Closes #64

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/server/package.json

@@ -66,7 +66,7 @@
     "@lezer/markdown": "^1.6.2",
     "@lezer/python": "^1.1.18",
     "@modelcontextprotocol/sdk": "^1.20.1",
-    "@opencode-ai/sdk": "^1.1.12",
+    "@opencode-ai/sdk": "1.2.6",
     "@sctg/sentencepiece-js": "^1.1.0",
     "@xterm/headless": "^6.0.0",
     "ai": "5.0.78",