ci: run full E2E suite on every push to main

[Hweinstock]

Problem

The full E2E test suite only runs on a weekly cron schedule (Mondays at 9 AM EST). Regressions merged to main can sit undetected for up to a week before the next scheduled run surfaces them.

The PR-level E2E runs a partial suite gated on authorized users, so external contributions can merge with no E2E coverage at all.

Risks

• A breaking change merges Monday afternoon, ships in a release Tuesday, customers report it Wednesday, weekly E2E catches it the following Monday
• Multiple regressions stack up between weekly runs, making root cause harder to isolate

Acceptance Criteria

• Full E2E suite runs on every push to main
• Concurrency controls prevent parallel E2E runs from conflicting
• No changes to the PR-level or weekly schedule triggers

[Hweinstock]

Resolved by #585