* Add linked issue guidance and ELI5 sections to PR generation prompts
Include linked GitHub issues in PR descriptions with Fixes/Refs guidance, and require ELI5 Problem and Solution sections before implementation details. Tests verify linked issue substitution and prompt structure enforcement.
* Include linked issue details in PR description generation
- Fetch the linked GitHub/GitLab issue title and body so generated PRs reference real issue context instead of just a number
- Use provider-specific reference syntax (Fixes/Refs, Closes/Related to, AB#) and label the issue by the active provider
- Feed issue title and description into the generation prompt while treating them as untrusted context, never as instructions
- Fall back to a cached work-item title when the provider lookup fails, and skip cross-provider issue attachment