disable automerge when adding existing PRs to stack

[skarim]

Disable auto-merge on existing PRs during submit

When gh stack submit discovers an existing PR for a branch, that PR may have auto-merge enabled. Auto-merge is incompatible with stacked PRs because the PR would merge independently, breaking the stack's base chain.

Previously, users could work around the link command's auto-merge guard by running init on a branch with an auto-merge-enabled PR, then submit to add it to a stack. The submit command had no auto-merge check, so the PR was adopted without any validation.

This change adds auto-merge detection to submit's ensurePR function. When an existing PR with auto-merge enabled is found, the
CLI automatically disables auto-merge via the disablePullRequestAutoMerge GraphQL mutation and warns the user. If the disable call fails, submit continues with a warning (non-fatal).

Changes

• internal/github/github.go: add DisableAutoMerge() method using the disablePullRequestAutoMerge GraphQL mutation
• internal/github/client_interface.go: add DisableAutoMerge to ClientOps interface
• internal/github/mock_client.go: add DisableAutoMergeFn field and mock implementation
• cmd/submit.go: in ensurePR, detect auto-merge-enabled PRs and disable auto-merge before adding to the stack
• cmd/submit_test.go: 3 new tests — auto-merge disabled successfully, disable failure continues with warning, and no-op when auto-merge is not enabled

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[Copilot]

Pull request overview

This PR closes a loophole where users could bypass the link command's auto-merge guard by running init on a branch with an auto-merge-enabled PR and then submit to adopt it into a stack. The fix adds auto-merge detection and automatic disabling during submit's PR reconciliation phase.

Changes:

• Adds a DisableAutoMerge GraphQL mutation method following the same pattern as MarkPRReadyForReview
• Integrates auto-merge detection in ensurePR as a non-fatal check that warns the user when auto-merge is disabled
• Includes three test cases covering success, failure (continues with warning), and no-op scenarios

Show a summary per file

File	Description
internal/github/github.go	New DisableAutoMerge() method using disablePullRequestAutoMerge GraphQL mutation
internal/github/client_interface.go	Adds DisableAutoMerge to the ClientOps interface
internal/github/mock_client.go	Adds DisableAutoMergeFn field and mock implementation
cmd/submit.go	Detects and disables auto-merge on existing PRs in ensurePR
cmd/submit_test.go	Three new tests covering all auto-merge disable scenarios

Copilot's findings

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• Files reviewed: 5/5 changed files
• Comments generated: 0

[ktravers]

Do we want to force the user to disable auto merge themselves? Do we foresee anyone getting grumpy about us disabling it for them without asking for confirmation first?

And actually, how tricky would it be to get user confirmation before disabling? That could be a nice prompt to add.

[Lukeghenco]

🚢 it!

[Lukeghenco]

Do we want to force the user to disable auto merge themselves? Do we foresee anyone getting grumpy about us disabling it for them without asking for confirmation first?

And actually, how tricky would it be to get user confirmation before disabling? That could be a nice prompt to add.

This isn't a bad idea. It would be nice to confirm you are ok with it disabling before it disables auto merge. Like an eject button before we disable auto merge and add the PR to the stack.