feat(pr): add --auto-merge-method to choose merge method

[julienduchesne]

Summary

Adds a new --auto-merge-method flag (env: GITHUB_AUTO_MERGE_METHOD) that controls which merge method is used when --auto-merge is enabled. Accepts merge, squash, or rebase; defaults to merge.

--auto-merge remains a boolean flag, so existing usage is unchanged.

[phlope]

Looks good, one behaviour i wanted to check before approving:
If a merge method is selected where not permitted/explicitly disabled on a repository, what is the result?

Looks like the change has coverage with the logger to surface the reason but i wanted to double check before 👍

[julienduchesne]

time=2026-04-27T02:02:26.452Z level=DEBUG msg="performing request" method=GET url=https://api.github.com/repos/grafana/deployment_tools/pulls/566458
time=2026-04-27T02:02:27.231Z level=DEBUG msg="performing request" method=GET url=https://api.github.com/repos/grafana/deployment_tools/pulls/566458
time=2026-04-27T02:02:27.962Z level=DEBUG msg="performing request" method=POST url=https://api.github.com/graphql
time=2026-04-27T02:02:28.538Z level=INFO msg="CI passed and auto-merge is enabled, merging PR"
time=2026-04-27T02:02:28.538Z level=DEBUG msg="performing request" method=PUT url=https://api.github.com/repos/grafana/deployment_tools/pulls/566458/merge
time=2026-04-27T02:02:29.457Z level=WARN msg="failed to merge PR, will retry on next poll" error="failed to merge PR: PUT https://api.github.com/repos/grafana/deployment_tools/pulls/566458/merge: 405 Merge commits are not allowed on this repository. []"
time=2026-04-27T02:02:29.457Z level=INFO msg=rechecking interval=30s

[julienduchesne]

Logs the error and retries 30 seconds later