fix: add loop detection to prevent agent from repeating identical actions

[octo-patch]

Fixes #112

Problem

The agent can get stuck in an infinite loop, repeatedly calling the same action with the same input (e.g., clicking the same button or entering the same text 4+ times in a row) without making progress. There was already a @todo loop detection comment in #handleObservations acknowledging this.

Solution

Added loop detection logic in PageAgentCore.#handleObservations(). After each step, the agent checks the last 4 consecutive history entries. If all 4 share the identical action name and input, a system observation is pushed into the agent's history:

⚠️ Loop detected: "click_element_by_index" has been called 4 times in a row with identical input. This approach is not working. Try a completely different strategy to achieve your goal.

This warning is included in the next LLM invocation as part of <agent_history>, prompting the model to reconsider its approach.

Testing

• Loop is triggered only after 4 consecutive identical actions (not 2–3, which may be intentional retries).
• Detection uses JSON.stringify(action.input) for deep equality on the action payload.
• Works for all tool types (click_element_by_index, input_text, scroll_down, etc.).
• Non-looping repeated scrolls (e.g., scroll 3 times to read content) are not falsely flagged.