refactor: overlay-based mutation and improved output handling

[sivchari]

Summary

• Replace file-based mutation approach with Go's -overlay flag for crash-safe testing
• Improve output handling: default to console summary, file formats write only to files
• Add progress indicators during mutation testing execution

Changes

Overlay-based Mutation (Crash-safe)

• New: internal/execution/overlay.go - OverlayMutator using Go's overlay feature
• New: internal/execution/overlay_test.go - Comprehensive tests for overlay functionality
• Removed: internal/execution/mutator.go - Old file-based mutation approach
• Removed: internal/execution/mutator_test.go - Tests for removed mutator
• Modified: internal/execution/engine.go - Updated to use overlay for compilation and test execution

Output Handling Improvements

• Default output: Changed from json to console (summary only to stdout)
• File formats: --output json/html/text now write to files without stdout spam
• Progress indicators: Show per-file processing status during execution
• Example output:

  Processing 3 file(s)...
  [1/3] foo.go (5 mutants) -> 3/5 killed
  [2/3] bar.go (no mutants)
  [3/3] baz.go (2 mutants) -> 2/2 killed
  
  Mutation Testing Results
  ========================
  Files: 2/3 processed
  Mutants: 7 total
  Killed: 5 (71.4%)
  ...
  

Other Fixes

• Fix wsl_v5 lint errors across multiple files
• Fix goreleaser deprecated properties (homebrew_casks.binary, conflicts.formula)
• Update README.md to reflect current CLI implementation

Benefits

• Crash-safe: Original files remain intact even if process is killed mid-execution
• True parallelism: Each mutation uses isolated overlay files, eliminating file contention
• Better UX: Clear progress during execution, clean output for file formats
• Simpler code: No file locks or rollback logic required

Test plan

• All existing tests pass with -race flag
• Parallel execution test (TestOverlayParallelExecution) verifies concurrent safety
• Linter checks pass
• goreleaser check passes

[github-actions]

🧬 Mutation Testing Results

✅ Quality Gate: PASSED

Overall Mutation Score: 24.6%
Total Mutants: 560
Killed: 138

Files with Mutations

File	Score	Mutants	Killed
...er/work/gomu/gomu/internal/execution/overlay.go	25.0%	100	25
...ner/work/gomu/gomu/internal/report/generator.go	51.0%	96	49
/home/runner/work/gomu/gomu/pkg/gomu/engine.go	21.4%	299	64
...ner/work/gomu/gomu/internal/execution/engine.go	0.0%	65	0

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Generated by gomu mutation testing