refactor(issue-261): complete sprint 2 helper cleanup lane

[graysurf]

Summary

• Implements issue Plan: Workspace obsolete test cleanup and de-risking #261 Sprint 2 lane tasks (S2T1-S2T5) in the assigned isolated worktree/branch.

Scope

• Added consolidated nils-test-support helper primitives for command execution, file writes, git repo setup, and git-cli runtime stubs.
• Adopted consolidated helpers in git-lock, semantic-commit, git-cli, and plan-issue-cli test harness modules.
• Reduced broad dead-code shielding by replacing crate-level allowances with targeted item-level allowances in selected test helper modules.
• Removed an unused image-processing CLI validation matrix that was guarded by dead-code allowances.
• Excluded: no non-Sprint-2 feature work and no CI workflow/pipeline changes.

Testing

• cargo test -p nils-git-scope -p nils-image-processing -p nils-api-testing-core -p nils-git-summary -p nils-plan-tooling (pass)
• cargo test -p nils-fzf-cli -p nils-semantic-commit -p nils-git-lock -p nils-memo-cli -p nils-agent-docs (pass)
• cargo test -p nils-test-support (pass)
• cargo test -p nils-git-cli -p nils-git-lock -p nils-semantic-commit -p nils-plan-issue-cli -p nils-macos-agent (pass)
• cargo test -p nils-git-cli -p nils-plan-issue-cli -p nils-macos-agent -p nils-screen-record -p nils-git-summary -p nils-fzf-cli (pass)
• cargo clippy -p nils-fzf-cli -p nils-semantic-commit -p nils-git-lock -p nils-memo-cli -p nils-agent-docs --tests -- -D warnings (pass)
• cargo clippy -p nils-test-support --tests -- -D warnings (pass)

Issue

• Plan: Workspace obsolete test cleanup and de-risking #261

WIP Handoff Notes (2026-02-26)

• This remains a draft PR intended as the Sprint 2 lane snapshot for issue Plan: Workspace obsolete test cleanup and de-risking #261 orchestration.
• No additional code changes were required in this handoff because the lane branch was already clean and in sync with origin.
• Focused smoke re-validation for this handoff:
  • cargo test -p nils-test-support --tests (pass)
  • cargo test -p nils-image-processing --tests (pass)

[github-actions]

Coverage

Total line coverage: 86.45% (58994/68243 lines hit).