Feature: per-job opt-in/opt-out for automatic AI error explanation

## Background

PR #201 introduced a global admin toggle (enableAutoExplainOnFailure) that automatically explains all failed builds via a RunListener. This is sufficient for the initial use case, but @panicking raised a valid point in #202: the feature should also be configurable at the job level.

## Problem

With only a global switch, there is no way to:

* Opt out a specific noisy job when the global toggle is enabled (e.g. a flaky test job that fails frequently and would waste AI quota)
* Opt in a specific job when the global toggle is disabled

## Proposed Solution

The `RunListener` would check the job-level property first, falling back to the global setting when the property is absent or set to inherit.

## Related
#202 (original feature request)
PR #201 (global toggle implementation)

Start by reviewing PR #201 and its RunListener implementation to identify how the global toggle is read. Define the job-level property and its inherit behavior, then verify that job opt-in and opt-out take precedence while absent or inherited settings fall back to the global toggle. Add or update coverage for each precedence case before considering the feature done.

Written by the indexing model from the issue text.