[codex] add Docker zellij e2e target

[dapi]

Summary

Adds the Docker zellij e2e target that was documented but missing from the repository.

Changes

• Add Dockerfile.e2e with pinned zellij 0.44.3 for amd64/arm64 and SHA256 verification.
• Add make test-e2e / docker-zellij-e2e to build Linux test binaries, build the image, and run the container.
• Add an isolated Docker runner that starts real zellij through script, runs sessions create, manually creates a Codex pane, runs sessions detect, and checks sessions list --live --json.
• Add a deterministic fake Codex Go binary that writes synthetic session_meta and stays alive for zellij pane discovery.
• Add the Docker e2e job to CI and update testing/development docs.

Validation

• make test-e2e
• go test ./...
• python3 scripts/check_memory_bank_index.py
• git diff --check HEAD~1..HEAD
• bash -n scripts/e2e/docker-runner.sh
• rg -n "zelima|Zelima" .

Notes

The original /Users/danil/code/zelma worktree still has unrelated unstaged markdown edits, so I did not force-pull it.