e2e-checkpoint-store: run all agents when the agent input is blank

Mirror e2e.yml: the agent input is now optional with a blank first option, and a matrix-setup job fans out to all agents when it's left empty (single agent when selected). The two jobs collapse into one matrix job — copilot-cli runs in the matrix with copilot-requests: write granted to every leg (harmless for the others) and COPILOT_GITHUB_TOKEN passed through, so the separate copilot job is no longer needed. fail-fast: false so one agent's failure doesn't cancel the rest. The checkpoint_store input (git-branch / git-refs) is unchanged and applies to every agent in the run.