(autofix) Explore context attachment with @

[aayush-se]

Some context we can't include automatically, but they could still help sometimes without us knowing. Make it easy for users to attach the context they want:

• other issues in the trace
• any file in the codebase
• any commit in the repo
• tags on the issue

Implementation ideas:

• simply write all trace connected issues, github commit titles, and file names into the state (cons: no attaching before run starts, potentially too much data to store in the state)
• new endpoint that runs when you type, only needing repo and issue information. Returns the list of matching context. Perhaps a functools.cache could help as well. This can all be done in sentry without seer. seer will still have to handle processing the supplemental data.