Seer Code Review: Branch/Target Filtering Configuration

### Problem Statement

Currently, Seer Code Review runs on every pull request regardless of the source or target branch. For teams with multi-stage promotion pipelines (e.g., feature → develop → staging → production), this results in Seer scanning the same code multiple times, once when new code is introduced, and again on each subsequent environment promotion merge.

This creates some pain points:

- Redundant processing
- AI/bot fatigue.

### Solution Brainstorm

A configurable filtering mechanism that lets teams control when Seer triggers based on source and/or target branch. So reviews only run when new code is actually being introduced, not on subsequent environment promotion merges.

### Product Area

Other

The issue names no files, tests, or entry points. Start by locating Seer Code Review's pull-request trigger and configuration handling, then determine how source and target branch filters should be represented and documented. Done means teams can configure those filters so reviews skip promotion merges while still running for intended pull requests.

Written by the indexing model from the issue text.