feat(git-service): implement fork-based push and GPG commit signing

[hossam-farid]

Description

RHCLOUD-50379

Implements a fork-based two-token push workflow so nacho-bot pushes branches to its own fork and creates cross-repo PRs on the upstream repo, and adds GPG commit signing so all commits show as "Verified" on GitHub.

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How to test locally

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Anything reviewers should know?

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Checklist

• Tests: new/updated tests cover the change
• API: spec updated if endpoints changed (or N/A)
• Migrations: backwards compatible if schema changed (or N/A)
• Dependencies: no known impact to dependent services
• Security: reviewed against secure coding checklist (or N/A)

AI disclosure

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⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: cdb3bd16-4fc7-4143-90b0-a67095692529

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[hossam-farid]

/retest

[hossam-farid]

/retest