Changes

fix(flaky-tests): apply issue mutation limit consistently (#4563)

## Summary
- move `issueMutationLimit` ownership into `GithubIssueManager` so
create/reopen/comment share one ranking path
- rank candidate cases inside the manager, use the highest-ranked
mutable case as the issue sample, and only comment mutable cases within
a shared issue key
- add regression tests for shared issue-key comment/create behavior plus
keep the existing reopen coverage green

## Testing
- `deno test` (from `tools/reporters/ci/flaky-tests`)
 (commit: 58dbaa4)