ci: fix push checks

[Rememorio]

Summary

• Run golangci-lint only for pull requests, where only-new-issues has the required diff context.
• Keep golangci-lint on latest while avoiding push-event failures from repository-wide historical lint issues.
• Add Codecov configuration that keeps patch coverage gated at 85% while making project coverage informational.
• Use platform-specific SO_REUSEPORT values and stop reuseport tests from panicking after setup failures.

Validation

• go test ./internal/reuseport -count=1 -v
• go test ./... -count=1
• cd extensions/websocket && go test ./... -count=1
• cd examples && go test ./... -count=1
• go run github.com/golangci/golangci-lint/cmd/golangci-lint@v1.64.8 run --new-from-rev=upstream/main
• GOOS=linux GOARCH=amd64 go test -c ./internal/reuseport

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.58308%. Comparing base (6b64c61) to head (8246619).

Additional details and impacted files

@@              Coverage Diff              @@
##                main         #33   +/-   ##
=============================================
  Coverage   78.58308%   78.58308%           
=============================================
  Files             48          48           
  Lines           4291        4291           
=============================================
  Hits            3372        3372           
  Misses           688         688           
  Partials         231         231           

Flag	Coverage Δ
unittests	78.58308% <ø> (ø)

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[WineChord]

LGTM