Feat | Add --concurrency flag to control parallel application processing

[dag-andersen]

Summary

• Adds a new --concurrency flag to control the maximum number of applications processed concurrently
• Default is 40 (same as previous hardcoded value)
• Setting to 0 allows unlimited concurrency (not recommended)

Changes

• cmd/options.go: Add --concurrency flag with default value of 40, environment variable CONCURRENCY
• pkg/extract/extract.go: Replace hardcoded const maxWorkers = 40 with configurable parameter
• cmd/main.go: Pass concurrency config to RenderApplicationsFromBothBranches()
• docs/options.md: Document the new option

Usage

# Use default (40 concurrent)
argocd-diff-preview --repo owner/repo --target-branch feature

# Limit to 10 concurrent applications
argocd-diff-preview --repo owner/repo --target-branch feature --concurrency 10

# Unlimited (not recommended)
argocd-diff-preview --repo owner/repo --target-branch feature --concurrency 0

# Or via environment variable:
CONCURRENCY=10 argocd-diff-preview --repo owner/repo --target-branch feature