feat(issue-71): add parallel supervisor e2e coverage

[dapi]

Summary

• Adds an end-to-end e2e test that drives cmd/zelma through supervisor start-issue.
• Covers two parallel issue launches with unique zellij panes and independent review/fix/merge simulation.
• Keeps the change scoped to the e2e harness; no production logic changed.

Validation

• MISE_TRUSTED_CONFIG_PATHS="$PWD" mise exec -- go test ./internal/e2e -run 'TestIssueSupervisor(StartIssueReviewFixCleanupE2E|ParallelStartIssueE2E)$' -count=1
• MISE_TRUSTED_CONFIG_PATHS="$PWD" mise exec -- go test ./...
• python3 scripts/check_memory_bank_index.py
• git diff --check

Notes

• The new test uses a fake zellij fixture to simulate staggered startup and multi-pane orchestration.
• No runtime behavior was changed; this PR only adds acceptance coverage for FT-040.

Fixes #71
Issue: #71