Add --writeEmptyOutput flag on get-impacted-targets

[tinder-maxwellelliott]

Summary

Inspired by ewhauser/bazel-differ's --output-on-empty, this adds an opt-in --no-writeEmptyOutput flag on get-impacted-targets. When set, the command skips creating the output file entirely if no targets are impacted, so CI can branch on file existence instead of file contents:

bazel-diff get-impacted-targets ... --no-writeEmptyOutput -o impacted.txt
if [ -f impacted.txt ]; then
  bazel test --target_pattern_file=impacted.txt
fi

• Defaults to true (current behavior, always write).
• Only applies when -o/--output is set; STDOUT is unaffected.
• The interactor's execute / executeWithDistances are refactored into a pure compute* step + a write* step. The CLI now computes first and short-circuits before opening the output file when the set is empty and the user opted out. The original execute* entry points stay (now thin wrappers) so the existing tests and any external callers continue to work unchanged.

Background

This came out of a scheduled audit comparing our approach against three alternative differ projects. Findings:

• ewhauser/bazel-differ — small UX wins: --output-on-empty (this PR) and an end-to-end get-targets wrapper with a disk-backed hash cache. The rest of the fork is years behind ours on correctness (no bzlmod, naive external-repo collapse, no cquery mode, no version gating, debug Println left in production).
• bazel-contrib/target-determinator — one real correctness gap: configuration-aware hashing under cquery. That's a sizable refactor (key on (label, configuration) through TargetDigest/RuleHasher/BuildGraphHasher) and warrants a separate design discussion — filing a follow-up issue.
• friel-openai/bazel-differous — explicit parity port aimed at matching us; nothing actionable. On bzlmod they're materially behind ours.

Test plan

• bazel test //cli:CalculateImpactedTargetsInteractorTest — passes (new computeImpactedTargets_returnsEmpty_whenStartAndFinalMatch covers the compute step's empty case).
• bazel test //cli:CalculateImpactedTargetsInteractorModuleQueryTest //cli:CalculateImpactedTargetsInteractorIssue335Test — both still pass; existing execute/executeWithDistances paths are intact.
• bazel test //cli:E2ETest --test_filter=testGetImpactedTargets_writeEmptyOutput — new E2E test exercises all three cases (empty diff + --no-writeEmptyOutput → no file; empty diff with default → empty file; non-empty diff with --no-writeEmptyOutput → file IS written).
• bazel test //cli:E2ETest --test_filter=testE2E$ — primary E2E still green.
• bazel test //cli:E2ETest --test_filter=testTargetDistanceMetrics — executeWithDistances path still green.
• CI: full Ubuntu+macOS × Bazel 7/8/9 matrix.

🤖 Generated with Claude Code