fix: pin mypy below 2.0 for Python 3.9

[konard]

Summary

• constrain the dev dependency to mypy>=1.13.0,<2.0 while the template targets Python 3.9
• add a project-metadata regression test that rejects mypy 2.x
• document the compatibility fix in a changelog fragment

Reproduction

A fresh pip install -e ".[dev]" previously resolved mypy 2.3.0. The new focused test then failed because mypy 2.0 was allowed by the dev requirement, matching the Python 3.9 compatibility warning reported in issue 44.

Verification

• fresh dev install resolves mypy 1.20.2
• mypy src succeeds against the configured Python 3.9 target
• pytest -v --cov=src --cov-report=xml --cov-report=term (64 passed, 100% package coverage)
• ruff check .
• ruff format --check .
• python scripts/check_file_size.py
• python -m build
• twine check dist/*
• secretlint scan
• git diff --check origin/main...HEAD

Fixes #44

[konard]

Working session summary

Implemented and finalized PR #47.

• Pinned mypy>=1.13.0,<2.0.
• Added a regression test preventing mypy 2.x while targeting Python 3.9.
• Added the required changelog fragment.
• Merged the latest main.
• Local suite: 64 tests passed with 100% package coverage.
• Ruff, mypy, file-size, build, Twine, and secret checks passed.
• Fresh CI/CD and Docs workflows passed.
• Branch is clean, pushed, and synchronized.
• PR title/body updated and marked ready for review.

💰 Cost estimation:

• Model: GPT-5.6 Sol
• Provider: OpenAI
• Public pricing estimate: $3.400940

📊 Context and tokens usage:

• 83.6K / 200K (42%) input tokens, 12.0K / 128K (9%) output tokens

Total: (83.6K + 2.0M cached) input tokens, 12.0K output tokens, $3.400940 cost

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

[konard]

🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

• Model: GPT-5.6 Sol
• Provider: OpenAI
• Public pricing estimate: $3.400940

📊 Context and tokens usage:

• 83.6K / 200K (42%) input tokens, 12.0K / 128K (9%) output tokens

Total: (83.6K + 2.0M cached) input tokens, 12.0K output tokens, $3.400940 cost

🤖 Models used:

• Tool: OpenAI Codex
• Requested: gpt-5.6-sol
• Thinking level: medium (~15999 tokens)
• Model: GPT 5.6 Sol (gpt-5.6-sol)

📎 Log file uploaded as Gist (1304KB)

• View complete solution draft log

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Now working session is ended, feel free to review and add any feedback on the solution draft.

[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