Refactor PR status checks to use typed GraphQL union types

[Copilot]

PullRequest.ChecksStatus() was incorrectly deserializing StatusContext and CheckRun as flat fields, causing test failures when the GraphQL API returns them as discriminated union types.

Summary

Details

• New PullRequestComplex type: Replaces direct State/Status/Conclusion fields on context nodes with proper StatusContext and CheckRun union sub-structs; adds PullRequestMinimal as a shared base
• Rewritten PullRequests query: Migrated from raw GraphQL string to typed githubv4/graphql structs; introduces PullRequestByNumber and PullRequestForBranch via reflection for generic PR lookups
• PullRequestDiff signature: Changed from *PullRequest to prNumber int — caller no longer needs a full PR struct to fetch a diff
• PullRequestAndTotalCount: Now holds []PullRequestComplex; old []PullRequest variant renamed to LegacyPullRequestAndTotalCount
• Test fixtures: Updated to match actual GraphQL response shapes — issueCount (was totalCount), nodes (was edges), projectCards (was projectcards)
• api/pull_request_test.go: Updated JSON payload to wrap status nodes as {"StatusContext": {...}} / {"CheckRun": {...}} union objects

Original prompt

Pull Request: #4

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