[build-tools] Add eas/posthog_annotation workflow function

[gwdp]

Why

Drop a labeled marker on PostHog charts from a workflow, so a deploy or release shows up as a line across every graph. Reasoning: Makes it easy to see whether errors spiked right after a ship.

How

Adds eas/posthog_annotation. POSTs to the PostHog annotations API with content and an optional date_marker (defaults to now). Same personal key as the flag and metric steps; the key needs the annotation:write scope.

ignore_error (default false) fails the step on error; set it true to warn and continue.

Test Plan

CI passes. Not live-run yet, since the connect key needs the annotation:write scope. Will run it before undrafting. Test workflow below.

Test workflow

name: PostHog annotation test

jobs:
  annotate_now:
    environment: production
    steps:
      - uses: eas/posthog_annotation
        with:
          content: Test annotation from an EAS workflow

  annotate_dated:
    environment: production
    steps:
      - uses: eas/posthog_annotation
        with:
          content: Release marker, back-dated
          date_marker: 2026-07-03T00:00:00Z

[linear-code]

ENG-21551

[gwdp]

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

• [build-tools] Add eas/posthog_wait_for_query workflow function #3949
• [build-tools] Add eas/posthog_annotation workflow function #3948 👈 (View in Graphite)
• [build-tools] Add eas/posthog_upload_sourcemaps workflow function #3947
• [build-tools] Add eas/posthog_wait_for_metric workflow function #3945
• [build-tools] Add eas/posthog_flag_rollout workflow function #3944
• [build-tools] Add eas/posthog_capture_event workflow function #3934
• main

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.73%. Comparing base (d089cc8) to head (d70a085).

Additional details and impacted files

@@                             Coverage Diff                              @@
##           gwdp/eng-21551-posthog-upload-sourcemaps    #3948      +/-   ##
============================================================================
+ Coverage                                     59.71%   59.73%   +0.03%     
============================================================================
  Files                                           943      944       +1     
  Lines                                         41579    41601      +22     
  Branches                                       8758     8760       +2     
============================================================================
+ Hits                                          24826    24848      +22     
  Misses                                        16659    16659              
  Partials                                         94       94              

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[github-actions]

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[sjchmiela]

nit: let's use UserError and add tracking codes?

[gwdp]

handled in the client refactor, everything throws UserError with codes now (EAS_POSTHOG_MISSING_CREDENTIALS here, EAS_POSTHOG_FORBIDDEN / EAS_POSTHOG_REQUEST_FAILED from the shared requestAsync). thanks for the approve 🙏

[github-actions]

✅ Thank you for adding the changelog entry!