Support manual Git-backed history promotion and document consumer integration

[tkilias]

Goal

Support manual promotion of validated benchmark history and document integration for consuming projects.

Scope

• Let store stage and commit only generated history files when EXASOL_BENCHMARK_COMMIT_RESULTS=1.
• Require existing Git identity and never push.
• Document the external workflow sequence: direct pytest execution, package, upload, download, compare, then manual store.
• Document GITHUB_TOKEN compatibility.
• Do not create or modify GitHub workflows in this repository.

Acceptance Criteria

• Git behavior is mocked and verifies opt-in-only commits and scoped staging.
• Git errors are actionable.
• Documentation has copy-ready external workflow command examples.
• No workflow, reusable workflow, or dynamic matrix is added here.

Dependencies

• Depends on Store and load benchmark history collections #15 and Implement benchmark comparison and history storage CLI commands #19.