[Feature]: add an opt-in fail-fast policy for automatic command sequences

Feature request

Add an opt-in configuration flag that stops a configured automatic command sequence after the first failed command.

The proposed configuration is:

[config]
auto_command_stop_on_failure = true

When enabled, configured pr_commands, push_commands, and reviewer_commands should stop when PRAgent.handle_request() returns False or raises an exception.

The policy should apply consistently to the GitHub App, GitHub Action synchronize flow, GitLab, Bitbucket App/Server, Azure DevOps Server, and Gitea automatic command paths.

Motivation

PR-Agent currently continues running later configured commands after an earlier command fails. For example, a failed /review can be followed by /improve, producing additional work or comments based on an incomplete run.

The default must remain unchanged: when the option is disabled or absent, PR-Agent should continue processing the remaining commands as it does today.

This feature is intentionally opt-in. It should not retry or roll back commands that already completed.