tui: show Discord bug report banner only for beta channel users

The Discord bug report banner in the session header now only appears
for beta channel installations instead of all preview channels. This
ensures only beta testers see the bug report prompt, making it more
targeted and relevant to users actively testing pre-release builds.

packages/opencode/src/cli/cmd/tui/routes/session/index.tsx

@@ -232,7 +232,7 @@ export function Session() {
       `  █▀▀█  ${UI.Style.TEXT_DIM}${title}${UI.Style.TEXT_NORMAL}`,
       `  █  █  ${UI.Style.TEXT_DIM}opencode -s ${session()?.id}${UI.Style.TEXT_NORMAL}`,
     ]
-    if (Installation.isPreview()) {
+    if (Installation.CHANNEL === "beta") {
       lines.push(
         `  ▀▀▀▀  ${UI.Style.TEXT_DIM}found a bug? report it on discord at https://opencode.ai/discord${UI.Style.TEXT_NORMAL}`,
       )

packages/opencode/src/installation/index.ts

@@ -53,6 +53,10 @@ export namespace Installation {
     return CHANNEL !== "latest"
   }
 
+  export function isBeta() {
+    return CHANNEL === "beta"
+  }
+
   export function isLocal() {
     return CHANNEL === "local"
   }