[trissim]

Test fixtures wrote git identity with git config, which writes whatever repository git resolves. When GIT_DIR leaked in (git bisect run from a linked worktree, or a hook), that was the shared agent-comms .git/config. On 10-23 this put "Citation fixture" plus commit.gpgsign=false there, and "Ratchet test" too, which 106 real commits then carried. The conftest GIT_* scrub guards the environment, not the writer. This fixes the writer: no test writes identity or signing config any more.

The owner: tests/fixture_git.py declares the fixture identity once (user.name, user.email, commit.gpgsign). It derives two forms from that:

• IDENTITY: the -c arguments for one git command.
• ENVIRONMENT: the same settings through git's GIT_CONFIG_COUNT channel, for the git that code under test starts.

Sites fixed

• tests/guards/test_debt_ratchet.py: two config user.* writes deleted. The runner passes IDENTITY on every command, and the per-commit -c commit.gpgsign=false folded into it.
• tests/source_revision_fixture.py: config user.name, user.email and commit.gpgsign deleted. SourceRepository overrides Repo._run, the one git subprocess, to carry IDENTITY, which covers commit-tree and the tests that commit through source.git. git_input passes IDENTITY too.
• tests/test_carry.py: two config user.* writes deleted; the runner passes IDENTITY.
• tests/test_checkpoint_command.py: two config user.* writes deleted. The checkpoint command commits through its own git (cli_commands.py:1889), so an autouse fixture sets ENVIRONMENT with monkeypatch.
• Hand-copied -c user.name=... identities folded into IDENTITY on the runner in test_proof_build, test_checkout_submodules, test_order_progress, test_installed_merge and test_phase_evidence.

Proof

• Leak: a scratch bare repository was set as GIT_DIR and the fixtures were run directly, bypassing the conftest scrub.
  • origin/main's fixtures left user.name=Ratchet test, user.email=ratchet@example.invalid and commit.gpgsign=false in it.
  • This branch's fixtures left no user.* or gpgsign key.
• Tests: run with -p no:xdist and HOME set to an empty directory, so no global identity can stand in.
  • 131 tests across the touched files and every user of source_revision_fixture passed.
  • test_install_announcements::test_failed_rehearsal_does_not_publish_to_the_source_bus failed only because the empty HOME had no .local. It passes with the real HOME.
• debt_ratchet: 163 of 167 pass. Four test_clean_changed_lines_cannot_be_offset_with_a_deletion cases (FrozenFieldWrite and BannedVocabulary, under src/agent_comms and src/toad) fail with assert 0 == 1, and fail identically with origin/main's copy of the file. That red is already on main, and no open PR touches the ratchet.

Left alone: these sites already pass identity per command, so they cannot leak. They are not folded here because open PRs touch the same files: test_bus (#2795), test_install_rehearsal (#2556), test_storage_rules (#2801), test_storage_violations (#2469), test_unowned_checkout_work (#2801, #2469) and test_change_report (#2792).

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