Fix ES module imports for @actions packages

[Copilot]

The action fails at runtime with SyntaxError: The requested module '@actions/core' does not provide an export named 'default'. The @actions packages are CommonJS modules without default exports, incompatible with default import syntax in ES modules.

Changes

• Convert default imports to namespace imports for @actions packages:

  // Before
  import core from '@actions/core'
  import exec from '@actions/exec'
  import github from '@actions/github'
  import artifact from '@actions/artifact'
  
  // After
  import * as core from '@actions/core'
  import * as exec from '@actions/exec'
  import * as github from '@actions/github'
  import * as artifact from '@actions/artifact'

All usage of these modules remains unchanged as namespace imports expose the same API surface.

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