fix: support golangci-lint v2 JSON output

[FrankSpitulski]

Problem

The Go scanner still invokes golangci-lint run --out-format=json. golangci-lint v2 removed that flag, so every Go scan reports golangci_lint tooling unavailable (parser_error) and silently loses linter coverage.

Fix

Use the v2 --output.json.path stdout flag and disable stats so the existing JSON parser receives only the structured report. Add a focused regression test that executes the configured Go phase and verifies the exact command.

Validation

• Focused Go plugin suite: 12 passed
• Ruff check and format: passed
• Patched scanner against dsx-mcp test backend: completed without reduced golangci coverage
• Core suite: 5,649 passed and 153 skipped. Five pre-existing failures remain in Bash unused-import and review prompt tests, outside these files.