Remove git-agent CLI utility replaced by prefect worker

[Copilot]

The git-agent CLI entry-point and associated code has been replaced with the prefect worker system as mentioned in issue #5584. This PR completes the removal of the deprecated CLI utility while preserving all underlying git functionality.

Changes Made

Removed Files

• backend/infrahub/cli/git_agent.py - The CLI module itself
• docs/docs/reference/infrahub-cli/infrahub-git-agent.mdx - CLI documentation

Updated Files

• backend/infrahub/cli/__init__.py - Removed git_agent import and CLI integration
• backend/tests/unit/cli/test_cli.py - Removed test_git_agent_app test
• tasks/docs.py - Removed git-agent from documentation generation

What's Preserved

• GIT_AGENT component type - Still used by prefect workers for git operations
• All git functionality - Now handled via prefect worker system instead of standalone CLI
• Git-related user authentication - Config references to git-agent accounts preserved

Migration Path

The functionality previously provided by:

infrahub git-agent start

Is now handled automatically by the prefect worker system, providing better reliability, error handling, and supervision as described in the 1.1.0 release notes.

This completes the migration started in PR #6026.

Fixes #5584.

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[codspeed-hq]

CodSpeed Performance Report

Merging #6755 will not alter performance

_{Comparing copilot/fix-5584 (236df44) with stable (69b9ef5)}

Summary

✅ 10 untouched benchmarks