claude: add roachtest failure investigation command

[dt]

This is an attempt to have AI assist with roachtest failure investigation. AI seems too eager to jump to conclusions to let it try on its own, but it can automate data collection tasks and present information to a human detective to explore further.

This command was written by asking Claude to reflect on where it made mistakes and modify its instructions through repeated iterations on a few test-case tickets. This process revealed an extreme propensity to jump to often incorrect conclusions, thus the heavy emphasis on sticking to rigid rules around role and how to perform the task, including explicitly controlling the todo list.

Release note: none
Epic: none

[cockroach-teamcity]

This change is 

[dt]

The referenced guides with the actual technical details of various tests and how to read the logs are not claude-specific, but rather are just general technical detail on and strategies for inspecting a roachtest, so I put them in their own folder in their own PR to keep this one focused on just slowly repeating myself at claude.