Trace E2E: verify multiple agents coexist in one repository #3717

Problem

The compatibility matrix marks "More than one agent in one repository" as Unknown for every provider. The local E2E suite has no scenario proving that two distinct initialized providers retain their own hooks, sessions, commit trailers, and attribution in one workspace.

Scope

Add one shared E2E scenario and check for two distinct providers. Start with Pi plus one pinned supported provider, then update only the cells actually measured.

Acceptance criteria

• Initialize two distinct providers in one workspace without overwriting either hook configuration.
• Run separate sessions and commits for both providers.
• Assert distinct stored session IDs and correct provider identities.
• Assert each commit trailer names only its contributing session.
• Assert file attribution does not leak across provider sessions.
• Preserve existing user hook configuration for both agents.
• Add deterministic runner/workspace/check regressions without fixed sleeps.
• Run the live two-provider scenario and update only measured matrix cells.

Non-goals

• Treating two Pi sessions as proof of multi-provider coexistence.
• Changing production code unless the scenario exposes a separately diagnosed defect.
• Claiming support for provider pairs that were not run.

References

• E2E matrix design: Trace: end-to-end test matrix for agent harnesses #3635.
• Baseline Pi support: feat(trace): add Pi provider support #3704.