feat: add --exclude-ext flag to ignore changed files by extension

[vanandrew]

Closes #10.

Summary

• New repeatable --exclude-ext CLI flag (and exclude-extensions action input) that drops files from the diff by extension before any downstream logic. A docs-only change no longer fans out to packages or triggers test_all.
• Filter is applied right after git diff, so it works uniformly across single-lock, multi-lock, root triggers, and --detailed mapping.
• Extension input is forgiving: md, .md, MD all normalize to .md. Only the final suffix is matched (foo.tar.gz → .gz).

Test plan

• Unit tests for normalize_extensions and filter_excluded_extensions (case-insensitivity, leading-dot tolerance, no-extension files preserved, last-suffix-only matching)
• CLI run()-level tests for: docs-only inside a package (no affected), docs-only at .github/ (no test_all), and mixed .md+.py diff (only .py package affected)
• Action workflow matrix entries exclude-ext-single (single value, docs-only on a package with transitive dependents → empty) and exclude-ext-multi (comma-separated .md,.txt, mixed diff → only the .py package)
• Local pytest: 190 passed
• Smoke-tested the CLI on a fresh git repo: docs-only diff with --exclude-ext .md → no affected packages

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.16%. Comparing base (ea7e141) to head (3191682).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
+ Coverage   98.08%   98.16%   +0.08%     
==========================================
  Files           6        6              
  Lines         470      491      +21     
==========================================
+ Hits          461      482      +21     
  Misses          9        9              

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