docs: add code modification safety protocols to guidelines

Added 110 lines of safety guidelines to prevent future bugs:

New CRITICAL PROHIBITIONS:
- NEVER delete code without reading full context (20 lines before/after)
- NEVER make cascading fix-up edits without git diff review
- NEVER skip post-edit verification

New MANDATORY REQUIREMENTS:
- Post-Edit Verification (mandatory for all file modifications)
- Backup Before Large Changes
- Large Deletion Safety Pattern

New ERROR RECOVERY PROTOCOL:
- Immediate actions when mistakes occur
- Recovery examples with git commands
- Prevention learning points

New WORKFLOW CHECKLISTS:
- Error Recovery Protocol
- Phase Completion Verification

These guidelines address the critical bug where auth functions were
accidentally deleted during cleanup, preventing recurrence.

PROJECT_GUIDELINES.md

@@ -23,6 +23,45 @@
 - ❌ **NEVER force push to main/master** branches
 - ❌ **NEVER commit files with secrets** (.env, credentials.json, etc.)
 - ❌ **NEVER make assumptions** - ask clarifying questions when uncertain
+- ❌ **NEVER delete code without reading full context first** (minimum 20 lines before/after)
+- ❌ **NEVER make cascading fix-up edits without git diff review**
+  - After compilation error: STOP, review `git diff`, understand full impact
+  - Use revert/re-apply pattern instead of blind fixes
+- ❌ **NEVER skip post-edit verification** - must compile after each file edit
+
+### Cascading Fix-up Pattern (PROHIBITED)
+
+**WHAT NOT TO DO** - This caused critical bugs:
+```go
+// ❌ WRONG: Blindly making fixes after compilation error
+
+Edit 1: Delete deprecated code
+[Compilation error: undefined Register]
+
+Edit 2: Try to fix error (over-broad deletion)
+[More errors: undefined Login, GetProfile, etc.]
+
+Edit 3: Try to fix again (worse damage)
+[Even more errors: major functionality broken]
+```
+
+**CORRECT APPROACH**:
+```go
+// ✅ CORRECT: Stop, understand, then fix deliberately
+
+Edit 1: Delete deprecated code
+[Compilation error: undefined Register]
+
+STOP → Review git diff → Understand Register was accidentally deleted
+RESTORE → Get exact Register function from git history
+VERIFY → Compile successfully
+```
+
+**Key Principle**: When compilation errors occur after edits:
+1. STOP - Don't make more edits
+2. ANALYZE - Use `git diff` to understand what was changed
+3. RECOVER - Restore what was accidentally deleted/broken
+4. VERIFY - Compile and test before proceeding
 
 ---
 
@@ -80,6 +119,22 @@
 - ✅ If **.env is missing**, auto-generate secure values
 - ✅ Never commit secrets to repository
 
+### Code Modification Safety
+- ✅ **Post-Edit Verification (MANDATORY for ALL file modifications)**:
+  - Run `go build` for affected packages immediately after each edit
+  - Review `git diff filename` to verify only intended changes
+  - Validate functionality still works as expected
+  - Never proceed to next file until current edit is verified
+- ✅ **Backup Before Large Changes**:
+  - Create a stash: `git stash push -m "Pre-cleanup snapshot"` before removing >50 lines
+  - Or create a backup branch: `git branch backup-before-cleanup`
+  - This allows instant recovery if mistakes occur
+- ✅ **Large Deletion Safety Pattern**:
+  - Read at least 20 lines before/after deletion target
+  - Include unique identifiers in match (function signatures, specific comments)
+  - Use narrow matches - avoid generic patterns
+  - Verify line numbers match intended section
+
 ### Documentation
 - ✅ Update **README.md** when users/admins need to be informed
 - ✅ Document API changes with Bruno collections
@@ -111,6 +166,43 @@
 
 ---
 
+## 🔄 ERROR RECOVERY PROTOCOL
+
+When code modification mistakes occur (deleted wrong code, broke compilation, etc.):
+
+### Immediate Actions
+1. **STOP** - Don't make more edits
+2. **ASSESS** - What was deleted? Is it critical?
+3. **REVIEW** - Run `git diff` to see exact changes
+4. **RESTORE** - Choose appropriate recovery method:
+   - **Recent mistake**: `git checkout -- filename`
+   - **Complex restoration**: Use `git show HEAD:filename` to recover deleted code
+   - **Multiple files**: `git reset HEAD~1` (if safe)
+5. **VERIFY** - Compile and test restored code
+6. **DOCUMENT** - Note what went wrong for future reference
+
+### Recovery Examples
+```bash
+# Recover a deleted function from original file
+git show HEAD:internal/handlers/auth.go | sed -n '70,275p' > recovery.txt
+
+# Revert entire file to original state
+git checkout HEAD -- internal/handlers/auth.go
+
+# Use git stash to save current state before rollback
+git stash push -m "Broken state before fix"
+git checkout -- internal/handlers/auth.go
+```
+
+### Prevention (Learn From Mistakes)
+- Why did the mistake happen?
+- Was it too-broad matching?
+- Was it insufficient context reading?
+- Was it cascading fix-up attempts?
+- Update guidelines to prevent recurrence
+
+---
+
 ## 📋 WORKFLOW CHECKLISTS
 
 ### Before Making Frontend-Only Changes
@@ -155,6 +247,25 @@
 - [ ] Verify logical commit structure
 - [ ] Update README.md if user-facing changes
 
+### Error Recovery Protocol (If Code Mistakes Occur)
+- [ ] **Stop immediately** - don't make more edits
+- [ ] **Assess impact**: What was deleted? Is it critical?
+- [ ] **Review git diff**: See exact changes made
+- [ ] **Restore strategy**:
+  - If recent mistake: `git checkout -- filename`
+  - If complex: Reconstruct from git diff using `git show HEAD:filename`
+- [ ] **Verify recovery**: Compile and test restored code
+- [ ] **Document mistake**: Note what went wrong for future reference
+
+### Phase Completion Verification (Before Declaring "Complete")
+- [ ] All target code is removed/intact as intended
+- [ ] No unintended code was deleted
+- [ ] All affected files compile successfully
+- [ ] Run `go build ./...` for entire project
+- [ ] No critical functionality was broken
+- [ ] Git diff shows only intended changes
+- [ ] Review all modified files with `git diff --stat`
+
 ---
 
 ## 🏗 ARCHITECTURAL PATTERNS