refactor(topics): dedupe code_topics/alnum_key into path_topics

[nazarli-shabnam]

Summary

• gemini.py and heuristics.py each carried their own byte-for-byte copy of _code_topics()/_alnum_key() (and the CODE_EXTS constant), despite both files already importing shared helpers from path_topics.py.
• Moved both functions and the constant into path_topics.py as public code_topics() / alnum_key() / CODE_EXTS, and updated both call sites to import from there. Local variables that previously shadowed the function names (code_topics = _code_topics(...)) were renamed to file_code_topics to avoid shadowing the import.
• No behavior change — same logic, single source of truth.

Test plan

• pytest -q (unmodified)
• ruff check . / ruff format --check .

Closes #21

(Re-opened as a new PR: the original PR #29 was auto-closed by GitHub when main's history was rewritten to correct a contributor's commit attribution — this branch is unchanged.)

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 14841e3a-e96e-4f21-bf51-a14190334569

📥 Commits

Reviewing files that changed from the base of the PR and between 11a379a and ab52e93.

📒 Files selected for processing (5)

• .github/workflows/ci.yml
• git_explain/gemini.py
• git_explain/heuristics.py
• git_explain/path_topics.py
• pyproject.toml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch refactor/dedupe-topic-helpers

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