fix(automerge): use a repo-allowed merge method; surface enable_auto_merge errors

[toufali]

Fixes #109.

Problem

enable_auto_merge hardcodes mergeMethod: SQUASH, which fails on repos that disable squash merges (e.g. mozilla/fxa, merge-commit-only). approve_and_merge ignores the returned error and logs Auto-merge enabled regardless, so PRs are approved but never auto-merge — silently.

Fix

• github_utils.py: add _allowed_merge_method() — pick a method the repo permits (prefer SQUASH, else MERGE, else REBASE) and pass it to the mutation as a variable.
• automerge_dependabot.py: approve_and_merge now checks enable_auto_merge's return and raises SkipPR(...) with the real error, so a failure is reported honestly instead of logged as success.

Testing

Not run in the Actions context (needs app creds). Both files py_compile clean. Behavior on squash-enabled repos is unchanged (SQUASH is still preferred); merge-commit-only repos now fall back to MERGE.

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