Isolate E2E matrix entries on per-entry branches and reset failure state per run

[Copilot]

The nightly gh-aw matrix reported all entries as failed (52 failed test runs), but the failures were dominated by harness/reporting behavior rather than a single workflow defect. Matrix entries were sharing branch/failure state, causing noisy and misleading status reports.

• Problem summary

  • Matrix entries were running in ways that could inherit stale fails.txt state and depended on pushing compiled workflows through shared branch behavior.
  • This inflated per-entry failure reporting (including test-copilot-call-workflow) and made status output less actionable.

• Workflow execution isolation

  • Updated .github/workflows/e2e.yml to run each matrix entry on a dedicated branch derived from ref × samples:
    • e2e_branch="e2e-matrix-${slug}"
  • Passed --branch "$e2e_branch" to e2e.sh in both source and extension modes so compile/push/dispatch are scoped to that entry.

• Per-entry failure-state reset

  • Removed carry-over failure state at the start of each matrix entry:
    • rm -f fails.txt
  • Ensures per-entry report artifacts reflect only failures produced by that entry.

• Documentation/comments alignment

  • Updated in-file workflow comments and status-report narrative to describe dedicated-branch dispatch behavior instead of “push/dispatch from main”.

slug=$(echo "${E2E_REF}-samples-${E2E_USE_SAMPLES}" | tr '/' '_' | tr -cd 'A-Za-z0-9._-')
e2e_branch="e2e-matrix-${slug}"
rm -f fails.txt

./e2e.sh --gh-aw-ref "$E2E_REF" --branch "$e2e_branch" --workflow-dispatch-only --verbose $SAMPLES_FLAG
# extension mode:
./e2e.sh --branch "$e2e_branch" --workflow-dispatch-only --verbose $SAMPLES_FLAG