docs: explain GitHub service account identity for conversations

[rajshah4]

Summary

• Add a customer-facing service account workflow to the Enterprise GitHub guide.
• Clarify how the conversation PAT, saved Git name/email, and built-in resolver identities differ.
• Link the workflow from the conversation secrets guide and regenerate llms-full.txt.

Validation

• Reviewed the current public docs and the Enterprise conversation startup code.
• Ran git diff --check and regenerated the LLM docs index with make llms.

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Project	Status	Preview	Updated
all-hands-ai	🟢 Ready	View Preview	Sep 21, 2026, 10:15 PM

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[rajshah4]

@jpelletier1 @ak684 — could you review this when you have a chance? The PR is ready for review, mergeable, and its Mintlify/link checks are passing. It documents the Enterprise GitHub service-account workflow and distinguishes the conversation PAT, Git commit identity, and the built-in resolver. Preview: https://allhandsai-docs-github-service-account-identity.mintlify.site/enterprise/integrations/github