feat: cross-phase regression gate in execute-phase pipeline (#945) (#1120)

Add regression_gate step between executor completion and verification
in execute-phase workflow. Runs prior phases' test suites to catch
cross-phase regressions before they compound.

- Discovers prior VERIFICATION.md files and extracts test file paths
- Detects project test runner (jest/vitest/cargo/pytest)
- Reports pass/fail with options to fix, continue, or abort
- Skips silently for first phase or when no prior tests exist

Changes:
- execute-phase.md: New regression_gate step
- CHANGELOG.md: Document regression gate feature
- docs/FEATURES.md: Add REQ-EXEC-09

Fixes #945

Co-authored-by: TÂCHES <afromanguy@me.com>

CHANGELOG.md

@@ -12,6 +12,7 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
   - Pre-wave dependency check in `execute-phase`: verifies key-links from prior wave artifacts before spawning next wave
   - Cross-Plan Data Contracts (Dimension 9) in plan-checker: detects incompatible transformations between plans sharing data pipelines
   - Export-level spot check in `verify-phase`: catches dead stores that exist in wired files but are never called
+- **Cross-phase regression gate** — New `regression_gate` step in `execute-phase` runs prior phases' test suites after execution completes but before verification, catching regressions before they compound (#945)
 - **Requirements coverage gate in plan-phase** — New step 13 verifies all phase requirements are covered by at least one plan before planning completes. Cross-checks REQ-IDs from ROADMAP against plan frontmatter and CONTEXT.md features against plan objectives (#984)
 - **`/gsd:ship` command** — Native PR creation workflow that bridges local completion → merged PR. Auto-generates rich PR body from planning artifacts (SUMMARY.md, VERIFICATION.md, REQUIREMENTS.md), pushes branch, creates PR via `gh`, optionally requests review, and updates STATE.md with shipping status (#829)
 

docs/FEATURES.md

@@ -221,6 +221,7 @@
 - REQ-EXEC-06: System MUST run post-execution verifier to check phase goals were met
 - REQ-EXEC-07: System MUST support git branching strategies (`none`, `phase`, `milestone`)
 - REQ-EXEC-08: System MUST invoke node repair operator on task verification failure (when enabled)
+- REQ-EXEC-09: System MUST run prior phases' test suites before verification to catch cross-phase regressions
 
 **Produces:**
 | Artifact | Description |

get-shit-done/workflows/execute-phase.md

@@ -415,6 +415,67 @@ node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" commit "docs(phase-${PARENT
 ```
 </step>
 
+<step name="regression_gate">
+Run prior phases' test suites to catch cross-phase regressions BEFORE verification.
+
+**Skip if:** This is the first phase (no prior phases), or no prior VERIFICATION.md files exist.
+
+**Step 1: Discover prior phases' test files**
+```bash
+# Find all VERIFICATION.md files from prior phases in current milestone
+PRIOR_VERIFICATIONS=$(find .planning/phases/ -name "*-VERIFICATION.md" ! -path "*${PHASE_NUMBER}*" 2>/dev/null)
+```
+
+**Step 2: Extract test file lists from prior verifications**
+
+For each VERIFICATION.md found, look for test file references:
+- Lines containing `test`, `spec`, or `__tests__` paths
+- The "Test Suite" or "Automated Checks" section
+- File patterns from `key-files.created` in corresponding SUMMARY.md files that match `*.test.*` or `*.spec.*`
+
+Collect all unique test file paths into `REGRESSION_FILES`.
+
+**Step 3: Run regression tests (if any found)**
+
+```bash
+# Detect test runner and run prior phase tests
+if [ -f "package.json" ]; then
+  # Node.js — use project's test runner
+  npx jest ${REGRESSION_FILES} --passWithNoTests --no-coverage -q 2>&1 || npx vitest run ${REGRESSION_FILES} 2>&1
+elif [ -f "Cargo.toml" ]; then
+  cargo test 2>&1
+elif [ -f "requirements.txt" ] || [ -f "pyproject.toml" ]; then
+  python -m pytest ${REGRESSION_FILES} -q --tb=short 2>&1
+fi
+```
+
+**Step 4: Report results**
+
+If all tests pass:
+```
+✓ Regression gate: {N} prior-phase test files passed — no regressions detected
+```
+→ Proceed to verify_phase_goal
+
+If any tests fail:
+```
+## ⚠ Cross-Phase Regression Detected
+
+Phase {X} execution may have broken functionality from prior phases.
+
+| Test File | Phase | Status | Detail |
+|-----------|-------|--------|--------|
+| {file} | {origin_phase} | FAILED | {first_failure_line} |
+
+Options:
+1. Fix regressions before verification (recommended)
+2. Continue to verification anyway (regressions will compound)
+3. Abort phase — roll back and re-plan
+```
+
+Use AskUserQuestion to present the options.
+</step>
+
 <step name="verify_phase_goal">
 Verify phase achieved its GOAL, not just completed tasks.