Add branch information to Known Issue error report table

## Summary

The Known Issue error report table currently includes Build, Repository, Step Name, Console log, and Pull Request columns. Adding **branch information** would make the report more useful for prioritization and knowing what fixes to backport.

## Proposal

Add a **Branch** column to the Known Issue error report table. For example:

| Build | Branch | Repository | Step Name | Console log | Pull Request |
|---|---|---|---|---|---|
| [1482796](...) | main | dotnet/sdk | TestStep | [log](...) | dotnet/sdk#54904 |
| [3008697](...) | release/10.0.1xx | dotnet/sdk | TestStep | [log](...) | |

This could be derived from the PR's base branch or the build's source branch (for branch CI builds without a PR).

## Motivation

Knowing whether a known build error is occurring on `main` vs a servicing branch is critical for prioritization — it tells you which failures to tackle first, helps measure the impact of changes you've made to a specific branch, and makes it immediately obvious when a fix needs to be backported to servicing.

For product repos like dotnet/sdk, the Repository column is redundant since all known issues are filed in the same repo. Branch is far more useful information in that context — it's what actually differentiates one occurrence from another in a meaningful way.

Currently, determining the branch requires clicking through each build or PR link individually, which is tedious when there are dozens of entries.

Start by locating the Known Issue error report table and tracing the data used for each row. Determine whether branch comes from the pull request base or the build source branch, including builds without a pull request. Done means the table shows a Branch column with the correct value for both cases.

Written by the indexing model from the issue text.