[eas-build-job] Add local action schema

[sswrk]

Why

First step toward local actions for EAS Workflows: reusable step groups referenced via uses: in .eas/workflows/*.yml, similar to GitHub Actions.

This PR adds the shared contract in @expo/eas-build-job so all consumers agree on action config shape.

How

Introduce the action config schema in @expo/eas-build-job, the single source of truth for the shape of an action.yml file. No runtime behavior changes yet.

Added:

• ActionConfigZ / ActionConfig: Zod schema and type for action.yml (inputs, outputs, runs.steps)
• validateActionConfig: validates a raw action config against the schema

Test Plan

Added unit tests for validateActionConfig.

[linear-code]

ENG-22387

[sswrk]

• [eas-cli] Validate local actions in workflow files #3931
• [build-tools] Load local action catalog at build runtime #3930
• [steps] Expand local actions at parse time #3929
• [eas-build-job] Add local action schema #3928 👈 (View in Graphite)
• main

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.30%. Comparing base (a848411) to head (e64f37d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3928      +/-   ##
==========================================
+ Coverage   59.28%   59.30%   +0.03%     
==========================================
  Files         935      936       +1     
  Lines       41093    41114      +21     
  Branches     8660     8665       +5     
==========================================
+ Hits        24356    24377      +21     
  Misses      16643    16643              
  Partials       94       94              

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