fix: use 'mergeable' field for PR conflict detection

[bborn]

Summary

• Fixed PR merge conflict detection by using the correct GitHub API field
• Tasks with PRs that have merge conflicts now properly display the conflict indicator

Problem

The merge conflict detection feature was implemented but not working because:

• The code was querying GitHub's mergeStateStatus field (returns "DIRTY", "CLEAN", etc.)
• But checking for the value "CONFLICTING" (which only appears in the mergeable field)
• This meant PRs with conflicts were never detected

Solution

Updated all GitHub CLI queries to use the mergeable field instead of mergeStateStatus:

• Single PR queries: gh pr view
• Batch PR queries: gh pr list (both open and merged)
• Updated both JSON struct definitions and parsing code

Testing

• ✅ All existing unit tests pass (github and ui packages)
• ✅ Verified with actual PR feat: add PR URL/number search to ctrl+p command palette #140 which has conflicts: gh pr view now returns "mergeable":"CONFLICTING"

Changes

File: internal/github/pr.go

• Updated ghPRResponse struct to use Mergeable field
• Updated ghPRListResponse struct to use Mergeable field
• Changed all 3 GitHub CLI queries to request mergeable instead of mergeStateStatus
• Updated response parsing to use resp.Mergeable instead of resp.MergeStateStatus

User Impact

Users will now see:

• Red "C" badge on tasks with PRs that have merge conflicts
• "Has conflicts" status description in both Kanban board and detail view
• This status updates every 30 seconds with the PR refresh cycle

🤖 Generated with Claude Code

[openhands-ai]

Looks like there are a few issues preventing this PR from being merged!

• GitHub Actions are failing:
  • CI

If you'd like me to help, just leave a comment, like

@OpenHands please fix the failing actions on PR #143 at branch `task/286-task-should-reflect-pr-merge-conflicts-s`

Feel free to include any additional details that might help me get this PR into a better state.

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