fix(ui): unblock ask_user promise when Stop is clicked (fixes #183)

[octo-patch]

Fixes #183

Problem

When the agent invokes the ask_user tool and the user clicks Stop before typing an answer, the panel becomes permanently stuck:

1. Panel.#askUser() returns a Promise and stores only its resolve callback.
2. The LLM layer (OpenAIClient) awaits tool.execute(), which in turn awaits onAskUser() — without passing the abort signal to that inner await.
3. Clicking Stop calls agent.stop() → abortController.abort(), which cancels the outgoing HTTP request but has no effect on an already-running tool execution.
4. As a result, execute() stays blocked at await tool.execute() with status = 'running' forever.
5. The action button never morphs from ■ (stop) to X (close), so the user cannot close the panel.

Solution

Introduce #cancelUserAnswer() which rejects the pending #askUser promise with an AbortError-shaped error (error.name = 'AbortError').

The LLM retry layer already checks rawError.name === 'AbortError' to skip retries for user aborts, so the error propagates immediately to execute()'s catch block → #onDone(false) → status = 'error'. The panel button then becomes X, allowing the user to close it.

Changes in packages/ui/src/panel/Panel.ts:

• Add #userAnswerRejecter field alongside the existing #userAnswerResolver.
• Capture reject in #askUser().
• Add #cancelUserAnswer() — rejects the promise, clears state, and removes temporary question cards from the history panel.
• Call #cancelUserAnswer() in #handleActionButton() before agent.stop() when #isWaitingForUserAnswer is true.
• Call #cancelUserAnswer() in dispose() (after removing event listeners) to prevent memory leaks if the agent is disposed while waiting for user input.
• Clear #userAnswerRejecter in reset() and #handleUserAnswer().

Testing

1. Start a task that triggers the ask_user tool (e.g. configure the agent so the LLM asks a clarifying question).
2. When the question prompt appears in the panel, click ■ (Stop) without typing anything.
3. Before fix: the button stays ■ and the panel cannot be closed.
4. After fix: the button changes to X and clicking it closes the panel cleanly.