fix(github): respect automergeStrategy with platform automerge

[jetersen]

Changes

Use the configured automergeStrategy when enabling GitHub platform-native automerge, falling back to the repository-detected merge method when the strategy is auto or unsupported.

Add a regression test proving that an explicit rebase strategy overrides the detected squash method in the enablePullRequestAutoMerge GraphQL mutation.

Document that supported platforms apply automergeStrategy to platform-native automerge, with cross-references from both related option sections.

Context

Related discussion: #41177

• This closes an existing Issue, Closes: #
• This doesn’t close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI-generated non-trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Codex, based on GPT-5, was used to inspect the codebase, implement the change, add the regression test and documentation, and prepare this pull request description.

Documentation

• I have updated the documentation, or
• No documentation update is required

How I’ve tested my work

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests, but ran on a real repository, or
• Both unit tests + ran on a real repository

Full pnpm test using the repository-pinned Node 24.18.0 toolchain:

• 942 test files passed
• 24,195 tests passed; 1 expected failure
• 100% statements, functions, and lines coverage

The documentation updates also pass pnpm check --all docs/usage/configuration-options.md.

[cla-assistant]

All committers have signed the CLA.

[viceice]

needs docs update, check automerge strategy docs

[jetersen]

@viceice not sure which of the two commits you prefer for wording the docs, but let me know and I'll update it.

[viceice]

renovate/lib/config/options/index.ts

Line 2333 in 64e6a4c

supportedPlatforms: ['azure', 'bitbucket', 'forgejo', 'gitea', 'github'],

I thought it was missing here, but isn't.

[jetersen]

@viceice at least after checking who supports respecting automergeStrategy with platform automerge, here is the list:

• Azure, Gitea and Forgejo already passes automergeStrategy to native automerge.
• This PR brings GitHub into alignment with them.
• Bitbucket Server and GitLab support platformAutomerge but not automergeStrategy.

Getting Bitbucket Server and GitLab supported in a follow up PRs but I can't make promise on Bitbucket Server. But at least it would be good to align on the platform support.