feat(git): implement git audit module

[kaustuvbot]

Summary

Implements the complete git audit module for devopsctl, providing infrastructure hygiene checks for git repositories.

• Git client foundation (internal/git/client.go): lightweight wrapper for executing git commands safely
• Repository size check (internal/git/size.go): detects repos exceeding configured size thresholds
• Stale branch detection (internal/git/branches.go): flags branches with no recent commits
• Large file scanner (internal/git/files.go): identifies files exceeding size limits
• Git runner (internal/git/runner.go): orchestrates all git checks and returns []CheckResult
• CLI wiring (internal/cli/audit.go): devopsctl audit git command wired to runner
• Unit tests (internal/git/*_test.go): coverage for size check, stale branch detection, large file scanner, runner, and error handling

Test plan

• go build ./... passes
• go test ./... passes (all git package tests green)
• go vet ./... produces no warnings
• devopsctl audit git command runs without error

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