fix: exclude examples directory from code coverage reports

[miclle]

Summary

This PR fixes the code coverage reporting by properly excluding the examples/ directory from coverage calculations. Previously, the coverage badge showed 49% (Codecov) and 57.3% (local) due to including untested example code. After this fix, coverage accurately reflects ~90.6%.

Problem

• go test -coverprofile=coverage.txt ./... was including all files in the examples/ directory (191 lines of untested code)
• These example files had 0% coverage, dragging down the overall coverage from 90.6% to 57.3%
• While .codecov.yml configured to ignore examples, the coverage file already contained the data

Changes

1. Makefile (Makefile:47-48)

Added post-test filtering to remove examples from coverage.txt:
```bash
@echo "Filtering coverage to exclude examples..."
@grep -v "examples/" coverage.txt > coverage_filtered.txt && mv coverage_filtered.txt coverage.txt || true
```

2. GitHub Workflow (.github/workflows/go.yml:71-79)

Added dedicated filtering step before Codecov upload:
```yaml

• name: Filter coverage to exclude examples
  if: matrix.os == 'ubuntu-latest'
  run: |
  if [ -f coverage.txt ]; then
  grep -v "examples/" coverage.txt > coverage_filtered.txt || true
  if [ -s coverage_filtered.txt ]; then
  mv coverage_filtered.txt coverage.txt
  fi
  fi
  ```

3. Gitignore (.gitignore:24-26)

Added patterns to ignore temporary coverage files:
```gitignore
coverage*.txt
coverage*.html
coverage*.out
```

Impact

Metric	Before	After
Total Coverage	57.3%	90.6% ✅
coverage.txt lines	571	380
Examples data	191 lines (0% coverage)	0 lines ✅

Test Plan

• Local test: make test → Coverage shows 90.6%
• Verify examples excluded: grep -c "examples/" coverage.txt → 0
• CI will run and upload correct coverage to Codecov

Verification

After merge, the Codecov badge should update from 49% to approximately 90%.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.87%. Comparing base (d58c31a) to head (233d542).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #43       +/-   ##
===========================================
+ Coverage   49.30%   87.87%   +38.57%     
===========================================
  Files          23       16        -7     
  Lines        1294      726      -568     
===========================================
  Hits          638      638               
+ Misses        638       70      -568     
  Partials       18       18               

Flag	Coverage Δ
unittests	87.87% <ø> (+38.57%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 7 files with indirect coverage changes

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