chore: bump @opencode-ai/sdk to 1.18.3

Updates the SDK dependency across web, UI, and VS Code packages.
Refreshes the lockfile and package versions to match the release.
Adds unreleased changelog notes for the current set of user-facing changes.

packages/web/package.json

@@ -25,7 +25,7 @@
   "dependencies": {
     "@clack/prompts": "^1.1.0",
     "@octokit/rest": "^22.0.1",
-    "@opencode-ai/sdk": "1.17.18",
+    "@opencode-ai/sdk": "1.18.3",
     "@simplewebauthn/server": "13.3.1",
     "adm-zip": "^0.5.16",
     "better-sqlite3": "^12.10.0",