fail_level has no effect when filter_mode (default: added) filters out all issues

[softorwhite]

Problem

fail_level appears to have no effect when all golangci-lint findings are
filtered out by filter_mode (default: added). This is especially confusing
because:

1. The README recommends migrating from the deprecated fail_on_error to
   fail_level, implying it controls job failure.
2. golangci-lint still produces ##[error] annotations in the job log via
   stderr, so the user sees errors but the job succeeds.

Steps to reproduce

• action-golangci-lint version: v2.10.0
• reviewdog version: v0.21.0

- uses: reviewdog/action-golangci-lint@v2
  with:
    fail_level: warning
    # filter_mode defaults to "added"

• golangci-lint finds issues on lines that already existed (not added in the PR).
• The CI log shows ##[error] annotations for each finding.
• The job still succeeds because reviewdog's filter_mode: added filters out
  all issues before fail_level is evaluated.

Root cause

In reviewdog.go,
ShouldFail is only evaluated for diagnostics where check.ShouldReport is
true (i.e., diagnostics that survive the filter_mode filter):

if !check.ShouldReport {
    // skipped — never checked against failLevel
} else {
    shouldFail = shouldFail || w.failLevel.ShouldFail(check.Diagnostic.GetSeverity())
}

So when filter_mode: added removes all findings, shouldFail stays false
and reviewdog exits 0 regardless of fail_level.

Suggestion

• Documentation: Clarify in the README that fail_level only applies to
  issues that pass the filter_mode filter, and that the default added mode
  may silently suppress all failures.
• UX (optional): Consider logging a warning when golangci-lint reports
  issues but all are filtered out, so users understand why the job succeeded
  despite ##[error] annotations in the log.