[steps] Add local action discovery utilities

[sswrk]

Why

This is the first @expo/steps piece for local reusable workflow actions. Follow-up PRs will load these actions, validate them in workflow files, and expand uses: ./... at parse time. This PR adds the shared discovery helpers that will be needed for all of this.

How

• Adds localActions.ts:
  • ./-prefixed uses: reference parsing (parseActionReference, isActionReference)
  • Recursive action catalog building from a step list (buildActionCatalogFromStepsAsync), including cycle detection
  • Path helpers to resolve local action directoreis and ensure they stay within the project root
• Rejects local action references in .eas/build/*.yml custom build configs, since actions are workflow-only.

Test Plan

Added unit tests.

[linear-code]

ENG-22387

[sswrk]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [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
• [steps] Add local action discovery utilities #3956 👈 (View in Graphite)
• [eas-build-job] Add local action schema #3928
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[codecov]

Codecov Report

❌ Patch coverage is 94.33962% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.33%. Comparing base (06676aa) to head (c230ee8).

Files with missing lines	Patch %	Lines
packages/steps/src/utils/localActions.ts	95.75%	2 Missing ⚠️
packages/steps/src/index.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                                     Coverage Diff                                      @@
##           szymonswierk/eng-22387-eas-build-job-local-action-schema    #3956      +/-   ##
============================================================================================
+ Coverage                                                     59.28%   59.33%   +0.05%     
============================================================================================
  Files                                                           936      937       +1     
  Lines                                                         41101    41154      +53     
  Branches                                                       8660     8671      +11     
============================================================================================
+ Hits                                                          24364    24414      +50     
- Misses                                                        16643    16646       +3     
  Partials                                                         94       94              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[github-actions]

⏩ The changelog entry check has been skipped since the "no changelog" label is present.