agent: do not point at a peer branch when shared git is off

[hhh2210]

When shared Git is disabled, each agent's origin is a private submission remote. The coop prompt and peer-exit notice previously told agents to fetch origin/<peer> anyway. A messaging-on, shared-Git-off Click run followed that guidance and failed with unknown revision.

This change names a peer branch only when shared Git is enabled and the peer published it. Prompts distinguish the local graded submission from a branch the colleague can read. All four Git × messaging configurations retain git push / gh pr create submission guidance while recommending only the enabled communication channels. The gh shim's help and conflict warning no longer unconditionally recommend messaging.

The existing peer-exit fixtures now explicitly model a shared remote, preserving their published/unpublished recovery-path coverage. New regressions cover prompt rendering for all four configurations, no messaging-off peer notices, and the private-remote pointer. Existing real-Git shim tests also verify the revised guidance. The task, grading, and messaging protocol are unchanged.

Validation on macOS / Python 3.13.13:

• Full pytest tests/ -q: 457 passed, 63 skipped. Redis-dependent unit tests used a temporary loopback fakeredis.TcpFakeServer via TEST_REDIS_URL; this validates the connector path against fake Redis, not a live Redis service. Modal/Docker/GCP integration tests remain skipped by default.
• Tests used isolated Git global/system configuration to avoid the workstation's automatic tag signing. The real-Git shim suite passed all 27 tests.
• ruff check src/cooperbench/, ruff format --check src/cooperbench/, and mypy src/cooperbench/: pass.
• Focused lint/format checks for the visibility and peer-exit tests, sh -n for the shim, and git diff --check: pass.

No new live model rollout was run for this prompt correction. Hosted CI has not reported checks for this PR yet.

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[Copilot]

Copilot review overview

🟡 Changes recommended

Existing peer-exit tests appear incompatible with the new default behavior and must be updated before merging.

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Review effort: Lite
Findings: 1

Open (1)

• Default git behavior breaks peer exit agent tests · New

What changed in this PR

Updates collaboration prompts and peer-work pointers to avoid inaccessible peer branches when shared Git is disabled.

Changes:

• Gates shared-branch guidance on git_enabled.
• Clarifies local pull-request submission behavior.
• Adds peer-visibility regression tests.

File	Summary
tests/​agents/​mini_swe_agent_v2/​test_peer_visibility.py	Adds prompt and peer-pointer visibility tests.
src/​cooperbench/​agents/​mini_swe_agent_v2/​config/​coop.yaml	Makes collaboration guidance conditional on shared Git.
src/​cooperbench/​agents/​mini_swe_agent_v2/​agents/​default.py	Avoids suggesting inaccessible peer branches. Existing peer-exit fixtures require updates for the changed default behavior.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The channel-aware guidance is consistent with runtime behavior and has focused regression coverage.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Default git behavior breaks peer exit agent tests