Add flag system and auto-share functionality

- Add Flag module for environment variable configuration
- Implement OPENCODE_AUTO_SHARE flag to automatically share new sessions
- Update session creation to conditionally auto-share based on flag

🤖 Generated with [OpenCode](https://opencode.ai)

Co-Authored-By: OpenCode <noreply@opencode.ai>

packages/opencode/src/flag/flag.ts

@@ -0,0 +1,8 @@
+export namespace Flag {
+  export const OPENCODE_AUTO_SHARE = truthy("OPENCODE_AUTO_SHARE")
+
+  function truthy(key: string) {
+    const value = process.env[key]?.toLowerCase()
+    return value === "true" || value === "1"
+  }
+}