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>

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 |