Delete a merged PR's branch only in its own repository

[ppXD]

Summary

• git.merge_pr with deleteSourceBranch on GitHub used to delete heads/{head.ref} in the base repository even when the pull request's head lived in a fork. An outsider who named a fork branch after a real base branch (release, a teammate's feature) got that base branch deleted on merge. The provider now deletes only when the head repository id equals the base repository id (GitHubRepositoryProvider.IsHeadInBaseRepository). A fork's head, or a head whose fork has since been deleted, is kept and reported as SkippedFork.
• The cleanup no longer swallows or throws. Its outcome is reported on RemotePullRequestMergeResult and in the node's outputs as sourceBranchDeletion (NotRequested / Deleted / Requested / SkippedFork / Failed) and sourceBranchDetail, whether the delete was refused (for example a protected branch), a read failed, or the cleanup was cancelled. A refused delete counts as deleted only when a branch read answers 404. A failed read leaves the refusal standing. A cancel during the cleanup returns the merge with Failed: the detail says "cancelled before GitHub confirmed it" and does not claim the branch was or was not deleted. A refusal that arrives while a cancel is pending is still reported, not raised as a failed merge.
• GitLab does not have the defect: should_remove_source_branch is applied server-side to the merge request's own source project. The provider now reports it as Requested.

Test plan

• Unit MergeSourceBranchTests: real Octokit/NGitLab provider against a loopback forge (StubProviderHost). Rows: same-repository delete (4 DELETE answers); fork kept, with the fork still present and with it deleted; refused delete surfaced; PR read-back failure surfaced; refused delete with the branch read failing (502, 403) reported Failed, not Deleted; cleanup cancelled during the DELETE (answer lost, and refused), with the merge returned and no second DELETE; nothing read or deleted when no delete was asked for; GitLab sends the flag and deletes nothing itself
• Unit GitMergePullRequestNodeTests: output shape for every SourceBranchDeletion value; the output schema enum matches SourceBranchDeletion
• Unit GitHubWriteRetryTests: a retried merge still deletes and reports Deleted
• Mutations: widening the branch read-back's catch (NotFoundException) to ApiException, or dropping the cancellation clause, turns the new rows red
• Integration PullRequestMergeSourceBranchFlowTests (Postgres): 2/2
• Full unit suite: 12350 passed, 1 skipped; dotnet build: 0 errors