ci: enforce Go vet warning gate

[EndlessSeeker]

What this changes

• replaces the empty core lint job with a required Go vet run after the normal prebuild step
• removes the duplicate empty lint placeholder from the plugin workflow
• fixes all five findings reported by the enforced vet scope
• adds a reusable make lint.vet target covering cmd, pkg, registry, and conformance code

OpenSSF evidence

This provides an enforced FLOSS warning tool and a zero-warning baseline for the OpenSSF Best Practices warnings and warnings_fixed criteria. Broader heuristic linters can be enabled incrementally without weakening this required gate.

Verification

• make lint.vet
• go test ./pkg/kube ./registry/nacos/mcpserver ./test/e2e/conformance/tests -run ^$
• git diff --check

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4193   +/-   ##
=======================================
  Coverage        ?   50.76%           
=======================================
  Files           ?       88           
  Lines           ?    13410           
  Branches        ?        0           
=======================================
  Hits            ?     6808           
  Misses          ?     6154           
  Partials        ?      448           

Files with missing lines	Coverage Δ
pkg/kube/client.go	0.00% <ø> (ø)

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