Make drift check --silent report only actionable failures

[fiberplane-operator]

Summary

• change text-mode drift check --silent failure output to include only stale/broken doc blocks plus the summary
• keep passing --silent runs fully quiet and keep JSON failure payloads complete on stderr
• document the updated --silent behavior and add integration coverage for pass, failure, --changed, and JSON modes

Fixes #37

Verification

• PATH=/workspace/.tools/zig-0.16.0:$PATH zig fmt src/main.zig src/commands/lint.zig test/integration/lint_test.zig
• git diff --check
• PATH=/workspace/.tools/zig-0.16.0:$PATH zig build test
• manual smoke test with fresh/stale/broken docs for drift check --silent and drift check --silent --format json

Note: sandbox did not have Zig or mise installed; I downloaded Zig 0.16.0 to /workspace/.tools/zig-0.16.0 for verification.