config: record learning M233 - check for base file deletions before merge

Added Step 2b to git-merge-linear skill that detects when task branch
incorrectly deletes files from base branch (e.g., after bad rebase
conflict resolution). Checks .claude/cat/ and plugin/ paths specifically.

Author: cowwoc

.claude/cat/retrospectives/mistakes.json

@@ -3090,6 +3090,32 @@
         "catches_variations": true
       },
       "correct_behavior": "Use cat:render-diff skill: git diff ${BASE_BRANCH}..HEAD | \"${CLAUDE_PLUGIN_ROOT}/scripts/render-diff.py\" and present verbatim output"
+    },
+    {
+      "id": "M233",
+      "timestamp": "2026-01-24T12:15:00Z",
+      "category": "git_operation_failure",
+      "description": "Task branch 2.0-license-validation-server-api deletes files added by v2.0 base branch after rebase",
+      "root_cause": "Rebase or squash operation resolved conflicts incorrectly, removing base branch additions instead of preserving them",
+      "rca_method": "C",
+      "rca_method_name": "causal-barrier",
+      "prevention_type": "skill",
+      "prevention_path": "${CLAUDE_PROJECT_DIR}/plugin/skills/git-merge-linear/SKILL.md",
+      "pattern_keywords": [
+        "rebase",
+        "file-deletion",
+        "conflict-resolution",
+        "base-branch"
+      ],
+      "prevention_implemented": true,
+      "prevention_verified": false,
+      "recurrence_of": null,
+      "prevention_quality": {
+        "verification_type": "positive",
+        "fragility": "low",
+        "catches_variations": true
+      },
+      "correct_behavior": "Before merging, check for unexpected file deletions from base branch, especially in .claude/cat/ and plugin/ directories"
     }
   ]
 }

.claude/cat/retrospectives/retrospectives.json

@@ -1,6 +1,6 @@
 {
   "last_retrospective": "2026-01-23T19:15:39-05:00",
-  "mistake_count_since_last": 4,
+  "mistake_count_since_last": 5,
   "config": {
     "mistake_count_threshold": 10,
     "trigger_interval_days": 7

plugin/skills/git-merge-linear/SKILL.md

@@ -87,6 +87,44 @@ fi
 echo "Base branch has not diverged - safe to proceed"
 ```
 
+### Step 2b: Check for Base File Deletions (MANDATORY - M233)
+
+**CRITICAL: Even after rebase, incorrect conflict resolution can delete base branch files.**
+
+```bash
+# Check if task branch deletes files that exist in base branch
+DELETED_FILES=$(git diff --name-status "${BASE_BRANCH}..HEAD" | grep "^D" | cut -f2)
+
+if [[ -n "$DELETED_FILES" ]]; then
+  echo "WARNING: Task branch deletes files from base branch:"
+  echo "$DELETED_FILES"
+  echo ""
+
+  # Check if these are intentional deletions or rebase artifacts
+  # Files in .claude/cat/ or plugin/ directories are likely unintentional
+  SUSPICIOUS=$(echo "$DELETED_FILES" | grep -E "^(\.claude/cat/|plugin/)" || true)
+
+  if [[ -n "$SUSPICIOUS" ]]; then
+    echo "ERROR: Suspicious deletions detected in infrastructure paths:"
+    echo "$SUSPICIOUS"
+    echo ""
+    echo "These deletions are likely from incorrect rebase conflict resolution."
+    echo ""
+    echo "Solution: Re-rebase with correct conflict resolution:"
+    echo "  git reset --hard origin/${TASK_BRANCH}  # If remote has clean state"
+    echo "  # Or reset to merge-base and cherry-pick task commits"
+    echo "  git checkout ${BASE_BRANCH}"
+    echo "  git checkout -B ${TASK_BRANCH}"
+    echo "  # Then cherry-pick your actual task commits"
+    exit 1
+  fi
+
+  echo "Deletions appear intentional (not in infrastructure paths)"
+fi
+
+echo "No suspicious file deletions - safe to proceed"
+```
+
 ### Step 3: Squash Commits (if needed)
 
 ```bash
@@ -205,6 +243,15 @@ if [[ "$DIVERGED" -gt 0 ]]; then
   exit 1
 fi
 
+# Check for suspicious file deletions (M233)
+SUSPICIOUS_DELETIONS=$(git diff --name-status "${BASE_BRANCH}..HEAD" | grep "^D" | cut -f2 | grep -E "^(\.claude/cat/|plugin/)" || true)
+if [[ -n "$SUSPICIOUS_DELETIONS" ]]; then
+  echo "ERROR: Task branch deletes infrastructure files from base:" >&2
+  echo "$SUSPICIOUS_DELETIONS" >&2
+  echo "Likely incorrect rebase conflict resolution. Re-rebase with correct resolution." >&2
+  exit 1
+fi
+
 # Squash if multiple commits
 COMMIT_COUNT=$(git rev-list --count "${BASE_BRANCH}..HEAD")
 if [[ "$COMMIT_COUNT" -gt 1 ]]; then