workflows/autosolver: indicate acknowledgement of assignments

Sometimes the process of putting together a PR to solve an issue, or revising an existing PR based on comments, can take non-trivial time. It'd be useful to indicate to the user when a request to auto-solve has been received and work has begun/is in progress, or when a comment suggesting changes has been seen and is being considered. One potential idea is to use the ability to react to comments/issues on github with emoji, e.g. add a 👁️ emoji reaction when an issue or comment is first seen and in progress, and then a 👍 or 👎 if autosolver was able to satisfy the request or not (in addition to the comment it leaves when it opens a PR or cannot do so). This is particularly relevant for cases like code-review comments where it is useful to know if they've been noticed or not before commenting further.

Jira issue: CRDB-61807

Start by locating the workflows/autosolver entry points and the existing handling for received requests, review comments, and completion or failure messages. Trace how GitHub issues and comments are observed, then determine where acknowledgement reactions and final success or failure reactions belong. Done means users can see that work was noticed and whether autosolver completed the request, with coverage for the relevant behavior.

Written by the indexing model from the issue text.