feat(#225): Run tier1 E2E tests on PRs by default, full suite nightly#

Problem#

Every PR runs the full test:e2e suite, which is slow and expensive. Tier1 tests provide sufficient coverage for day-to-day PR validation.

Task / Link#

Closes #225

Changes#

• Modified ci.yml e2e job to run test:e2e:tier1 by default on PRs
• Added e2e-full label override: PRs with this label run test:e2e instead
• Preserved single-retry logic for both variants
• Added comment to e2e job explaining tier1 rationale and linking to full-suite workflow
• Created e2e-full.yml for nightly cron (03:00 UTC) and workflow_dispatch with optional ref input

Notes#

• test:e2e:tier1 must exist in agent-core-e2e/package.json — assumed by the plan
• Nightly failures in e2e-full.yml are independent of PR status checks and won't block merges

Testing#

• CI workflow logic verified by code review
• Label detection uses github.event.pull_request.labels which is only populated on pull_request events (already gated by if: github.event_name == 'pull_request')