[build-tools] Add eas/posthog_capture_event workflow function

[gwdp]

Why

First PostHog workflow step: send an event from a workflow (e.g. mark a deploy).

How

add eas/posthog_capture_event function; Uses the env vars integrations:posthog:connect already sets, or api_key/host inputs.

No distinct_id = anonymous event, so CI runs don't create person profiles. Failures fail the step; set ignore_error: true to log a warning and continue instead (same as upload_artifact).

Test Plan

CI passes. Ran this workflow against a local — all three jobs green, both events visible in the project's PostHog activity (anonymous one under the eas-workflow person), and the ignored failure logs a warning without failing the step:

name: PostHog capture test

jobs:
  anonymous_event:
    steps:
      - uses: eas/posthog_capture_event
        with:
          event: eas_workflow_test
          properties:
            source: coin-flip-workflow-test

  identified_event:
    steps:
      - uses: eas/posthog_capture_event
        with:
          event: eas_workflow_test_identified
          distinct_id: gwdp-local-test
          properties:
            source: coin-flip-workflow-test
            variant: identified

  ignored_failure:
    steps:
      - uses: eas/posthog_capture_event
        with:
          event: eas_workflow_test_ignored
          host: https://example.com
          ignore_error: true

Workflow succeeds

Data appears on my posthog organization

[linear-code]

ENG-21551

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.33%. Comparing base (9247f3c) to head (0d8006c).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3934      +/-   ##
==========================================
+ Coverage   59.18%   59.33%   +0.15%     
==========================================
  Files         935      937       +2     
  Lines       41075    41144      +69     
  Branches     8648     8671      +23     
==========================================
+ Hits        24306    24407     +101     
+ Misses      16674    16643      -31     
+ Partials       95       94       -1     

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[github-actions]

✅ Thank you for adding the changelog entry!