Suppress Git default-branch checkout warning in release workflow

[konard]

Summary

• Add workflow-level GIT_CONFIG_* runtime config to .github/workflows/release.yml so actions/checkout@v6 initializes with init.defaultBranch=main.
• Add a workflow contract test that requires the env block to be present before the first checkout action.
• Add a changelog fragment for the CI warning suppression.

Fixes #28

Reproduction

1. Run the release workflow on a runner where Git emits the Git 3.0 default-branch hint during git init.
2. Inspect the actions/checkout@v6 log before this change and observe the default initial branch warning.
3. The new contract test reproduces the missing workflow-level config by failing when the GIT_CONFIG_* env block is absent.

Verification

• .venv/bin/python -m pytest tests/test_workflows.py::test_release_workflow_sets_git_default_branch_before_checkout -q
• .venv/bin/python -m pytest
• .venv/bin/ruff check .
• .venv/bin/ruff format --check .
• .venv/bin/mypy src/
• .venv/bin/python scripts/check_file_size.py

[konard]

Working session summary

Implemented and finalized PR #31: #31

Changed:

• Added workflow-level GIT_CONFIG_* env in release.yml so actions/checkout@v6 sees init.defaultBranch=main before its internal git init.
• Added a regression contract test in test_workflows.py.
• Added a changelog fragment and removed the bootstrap .gitkeep.

Verified:

• Focused regression test fails before the workflow fix and passes after it.
• pytest: 44 passed
• ruff check .: passed
• ruff format --check .: passed
• mypy src/: passed
• python scripts/check_file_size.py: passed
• GitHub Actions run 28673736164 passed on commit 9456baf.

PR #31 is updated, pushed, marked ready for review, merge state is clean, and the local working tree is clean.

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This summary was automatically extracted from the AI working session output.

[konard]

🤖 Solution Draft Log

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💰 Cost estimation:

• Model: GPT-5.5
• Provider: OpenAI
• Public pricing estimate: $2.809585

📊 Context and tokens usage:

• 101.3K / 200K (51%) input tokens, 12.5K / 128K (10%) output tokens

Total: (101.3K + 1.2M cached) input tokens, 12.5K output tokens, $2.809585 cost

🤖 Models used:

• Tool: OpenAI Codex
• Requested: gpt-5.5
• Thinking level: max (~31999 tokens)
• Model: GPT-5.5 (gpt-5.5)

📎 Log file uploaded as Gist (1254KB)

• View complete solution draft log

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[konard]

🎉 Auto-merged

This pull request has been automatically merged by hive-mind.

• All CI checks have passed

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Auto-merged by hive-mind with --auto-merge flag