bug: parallel agent execution overwrites existing test files (3,743 lines lost)

[gandalf-at-lerian]

Problem

During ring:dev-cycle execution (Gates 0 RED-GREEN for sort mappers), 3 backend-engineer-golang agents running in parallel overwrote existing test files, deleting 3,743 lines of pre-existing tests unrelated to the migration.

Root Cause

Gate 0 (Implementation) dispatches agents per execution unit. When multiple tasks run in parallel and touch test files in the same package, each agent uses Write/Create which overwrites the entire file. There is no file locking, merge strategy, or baseline snapshot.

Invariant (from @Garzão)

Tests may ONLY be removed if the code being tested was removed. In all other scenarios, tests MUST be preserved — only modified to reflect changes in the code under test.

This invariant is not currently enforced anywhere in the dev-cycle or agent prompts.

Proposed Fix (3 layers)

Layer 1: Agent-level protection (minimal fix)

Add explicit instructions to backend-engineer-golang agent prompt:

• NEVER delete or overwrite existing _test.go files entirely. Use Edit tool to modify specific test functions, never Write/Create on existing test files.
• NEVER remove test functions unless the code under test was deleted in the same task.
• When adding tests, create task-scoped test files (e.g., sort_mapper_migration_test.go) rather than modifying shared test files.
• Before writing any test file, read the existing file first and preserve all existing test functions.

Layer 2: Pre/post-execution snapshot (defense in depth)

Before dispatching parallel agents in Gate 0, the orchestrator should:

1. Snapshot all _test.go files in affected packages (git stash or file copy)
2. After all agents complete, diff against baseline
3. If any test file lost lines vs baseline → revert that file and re-dispatch the offending agent sequentially

Layer 3: Parallel execution conflict detection

During Step 1 (initialization), the orchestrator should:

1. Analyze which files each task/subtask will likely touch (based on requirements + codebase structure)
2. If multiple tasks target the same package → force sequential execution for those tasks
3. Allow parallel execution only for tasks with non-overlapping file scopes

Affected Components

• dev-team/skills/dev-cycle/SKILL.md — orchestrator needs snapshot + conflict detection
• dev-team/agents/backend-engineer-golang.md — agent needs test preservation rules
• dev-team/skills/dev-implementation/SKILL.md — skill dispatching agents needs parallel safety
• Potentially all backend-engineer-* agents (same pattern applies to TypeScript)

Priority

High — data loss (test deletion) undermines the entire quality gate system. The dev-cycle has 5 testing gates (3-7) that become meaningless if tests are silently deleted during implementation.

Reported by

@Carlao-LERIAN in #the-ring-feedbacks