Fix PR base branch update on merge conflicts

[Phlogistique]

When a merge conflict occurs during automatic PR stack update:

• Don't change the PR's base branch (keeps the diff readable)
• Don't delete the merged branch (user can reference it during resolution)

After the user resolves conflicts and pushes:

• Update the PR's base branch to trunk
• Delete the old base branch only if no other conflicted PRs depend on it

Changes:

• main(): Track UPDATED_TARGETS vs CONFLICTED_TARGETS separately
• main(): Only update base and delete branch for successfully updated PRs
• Add has_sibling_conflicts() helper to check if other PRs share the base
• continue_after_resolution(): Update base branch and conditionally delete
• Update E2E tests to verify the new behavior
• Add Scenario 3: sibling conflicts test for branch deletion timing

[Phlogistique]

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[chatgpt-codex-connector]

Avoid deleting whatever base the PR currently has

In continue_after_resolution, the branch deletion uses OLD_BASE that was derived from the PR’s current base earlier in the function. If a user resolves conflicts and manually updates the PR base to main/trunk (or any other branch) before the synchronize run, OLD_BASE will be that branch and this code will attempt git push origin :<branch>. On repos without protected branches, that can delete the default branch. Consider persisting the original merged branch from the conflict event (or guarding against deleting default/base branches or when NEW_TARGET is empty) rather than deleting the current base blindly.

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