chore(ci): limit /app/prefix matrix variant to master merges

[rusackas]

SUMMARY

Running both app_root variants ("" and "/app/prefix") on every PR doubles the number of E2E jobs (12 Cypress + 4 Playwright) for a check that is primarily relevant at integration time. This restricts the /app/prefix variant to push events (master merges), halving PR E2E compute while preserving full coverage on merged code.

The change uses GitHub Actions' fromJSON() expression in the matrix definition so the value is a proper array at runtime:

app_root: ${{ github.event_name == 'push' && fromJSON('["", "/app/prefix"]') || fromJSON('[""]') }}

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A — CI-only change, no UI impact.

TESTING INSTRUCTIONS

• CI on this PR will show 6 Cypress + 2 Playwright jobs (PR-time count — only app_root: "").
• A master merge will still trigger the full 12 Cypress + 4 Playwright jobs (both app_root variants).

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

No functional code changes — CI workflow only.

[bito-code-review]

Bito Automatic Review Skipped - Files Excluded

Bito didn't auto-review this change because all changed files are in the exclusion list for automatic reviews. No action is needed if you didn't intend for the agent to review it. Otherwise, to manually trigger a review, type /review in a comment and save.
You can change the excluded files settings here, or contact your Bito workspace admin at evan@preset.io.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.21%. Comparing base (e40648d) to head (dd056f2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #40385   +/-   ##
=======================================
  Coverage   64.21%   64.21%           
=======================================
  Files        2592     2592           
  Lines      139167   139167           
  Branches    32312    32312           
=======================================
  Hits        89361    89361           
  Misses      48273    48273           
  Partials     1533     1533           

Flag	Coverage Δ
javascript	67.33% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 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.