Add Python AST-based git commit summarizer

[Copilot]

No existing tooling parsed Python source structure when summarizing commits — the prior workflow relied on OpenAI GPT. This adds a pure-Python, no-API-key alternative that uses ast to extract structural symbol changes per commit, grouped by day.

Core script: scripts/git_commit_summary.py

• Fetches before/after file content from git for each changed .py file and parses both with ast
• Extracts top-level and class-level functions/classes; reports added / modified / deleted / moved
• Filters noise: docstrings stripped before AST comparison, import-only files skipped, stub bodies (pass/...) not reported as "added"
• Move detection: symbol deleted from one file + added to another in same commit → reported as moved (old.py → new.py)
• Outputs plain text or JSON; usable as a CLI tool or imported as a library

# Text summary for a date range
python scripts/git_commit_summary.py --since 2025-01-01 --until 2025-03-01 --branch main

# JSON output (e.g. for downstream tooling)
python scripts/git_commit_summary.py --since 2025-01-01 --output json

Sample output:

============================================================
  2025-06-15  (3 commit(s))
============================================================
  [abc12345] Refactor decoder pipeline  <dev@example.com>
      added    class    'BayesianDecoder'  (src/decoder.py)
      modified function 'ZhangReconstructionImplementation.time_bin_spike_counts_N_i'  (src/reconstruction.py)
      moved    function 'helper'  (utils.py -> core/helpers.py)

GitHub Actions workflow: .github/workflows/python-ast-commit-summarizer.yml

• workflow_dispatch with start_date, end_date, branch, output_format, show_empty inputs
• Checks out full history, runs the script, uploads result as a downloadable artifact
• permissions: contents: read (least-privilege)

Tests: tests/test_git_commit_summary.py

32 unit tests covering symbol extraction, AST normalization, trivial-body detection, import-only file detection, per-file change detection, move resolution, and text/JSON formatting.

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