feat: clear the inline prompt input textarea after a successful response

After the user sends a message and the AI returns successfully, the
inline prompt's textarea is cleared so they can immediately type a
follow-up message without manually deleting the previous prompt.

This is a re-add of the clearInput feature from the reverted
fd30e53 commit, but WITHOUT the AI entry button position change (the
button stays in the native cell toolbar per c0bcbda).

Changes:
  - opencode_inline_prompt.ts: add clearInput() public method that
    sets this._textarea.value = ''.
  - opencode_cell_actions.ts: _handleResponse on a successful response
    calls this._prompt?.clearInput() after the history refresh, so
    the new conversation turn starts with a clean input.
  - test: 'clears the input textarea after a successful response'
    (types into the textarea, fires _handleResponse success, asserts
    textarea.value === '').

Verification: pytest 37/37, jest 35/35 (was 34, +1 new test).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/components/opencode_cell_actions.ts

@@ -204,6 +204,8 @@ export class OpenCodeCellActions extends Widget {
       if (notebookPath) {
         void this._refreshHistory(notebookPath);
       }
+      // Clear the input so the user can type a follow-up message.
+      this._prompt?.clearInput();
       Notification.info(
         `OpenCode 完成 (${resp.markdown.length} chars). Session: ${resp.sessionId.slice(0, 8)}…`
       );

src/components/opencode_inline_prompt.ts

@@ -211,6 +211,15 @@ export class OpenCodeInlinePrompt extends Widget {
     }
   }
 
+  /**
+   * Clear the user input textarea (called by the cell action after a
+   * successful response, so the user can type a follow-up message
+   * without manually deleting the previous prompt).
+   */
+  clearInput(): void {
+    this._textarea.value = '';
+  }
+
   /**
    * Render the current session's messages into the scrollable history
    * area. User messages are plain text; assistant messages are rendered