[eas-cli] Connect existing PostHog accounts via browser handoff

[gwdp]

Why

Existing PostHog users couldn't connect — eas integrations:posthog:connect dead-ended with a "not supported yet" message when the account's email already had a PostHog account.

How

On the existing-user (requires_auth) branch, open the approval URL in the browser, poll for the connection the website callback creates, then continue with the rest of connect. Non-interactive runs refuse with a clear message instead of hanging. Pairs with the universe stack (the startPostHogConnection / completePostHogConnection mutations + the /posthog/callback website scene).

Test Plan

• yarn test connect (added: browser handoff opens the URL + polls, non-interactive refusal, poll timeout).
• run connect with an email that already has a PostHog account, approve in the browser, confirm it links and continues.

[codecov]

Codecov Report

❌ Patch coverage is 95.58824% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.11%. Comparing base (efc78f4) to head (cb15ee2).

Files with missing lines	Patch %	Lines
...ckages/eas-cli/src/graphql/queries/PostHogQuery.ts	0.00%	2 Missing ⚠️
...s/eas-cli/src/graphql/mutations/PostHogMutation.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3895      +/-   ##
==========================================
+ Coverage   59.05%   59.11%   +0.06%     
==========================================
  Files         935      935              
  Lines       40959    41019      +60     
  Branches     8630     8643      +13     
==========================================
+ Hits        24185    24244      +59     
- Misses      16678    16679       +1     
  Partials       96       96              

☔ 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]

✅ Thank you for adding the changelog entry!

[github-actions]

Subscribed to pull request

File Patterns	Mentions
**/*	@douglowder

Generated by CodeMention

Warning: The preamble and epilogue options in commentConfiguration are deprecated. Use template instead.