fix: redirect ssh_retry diagnostic messages to stderr

**Root Cause**: The ssh_retry function was outputting diagnostic messages
to stdout, which were being captured along with the actual command output
when using command substitution. This caused the CURRENT_SHA validation
to fail because the captured output contained multiple lines:

  SSH Attempt 1 of 3
  5521f2e2c49ddf933f160e7e5b6da7799fe5c4aa
  ✅ SSH command succeeded on attempt 1

The regex validation `^[a-fA-F0-9]{40}$` failed on this multi-line output,
causing CURRENT_SHA to be set to "unknown", which triggered the "first
deployment" logic and skipped stack removal detection entirely.

**Impact**: Removed stacks were not being cleaned up because the detection
was always being skipped due to the invalid SHA format.

**Fix**: Redirect all diagnostic echo statements in ssh_retry to stderr
using `>&2`. This ensures only the actual command output is captured in
stdout, allowing proper SHA validation.

**Test Plan**:
- Verify deploy runs capture correct CURRENT_SHA
- Confirm stack removal detection no longer skips
- Validate removed stacks are properly cleaned up

Changes:
- Add >&2 to all echo statements in ssh_retry function (8 instances)
- Diagnostic messages now appear in logs but don't pollute stdout
- Command output captured cleanly for variable assignment

Follows systematic debugging process from superpowers:systematic-debugging.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: owine

.github/workflows/deploy.yml

@@ -267,30 +267,30 @@ jobs:
             local last_exit_code=1
 
             while [ $attempt -le $max_attempts ]; do
-              echo "SSH Attempt $attempt of $max_attempts"
+              echo "SSH Attempt $attempt of $max_attempts" >&2
               if eval "$ssh_cmd"; then
-                echo "✅ SSH command succeeded on attempt $attempt"
+                echo "✅ SSH command succeeded on attempt $attempt" >&2
                 return 0
               else
                 last_exit_code=$?
-                echo "❌ SSH command failed on attempt $attempt (exit code: $last_exit_code)"
+                echo "❌ SSH command failed on attempt $attempt (exit code: $last_exit_code)" >&2
 
                 # Check for specific SSH errors
                 case $last_exit_code in
-                  255) echo "SSH connection error - network/auth issue" ;;
-                  1) echo "General SSH error" ;;
-                  *) echo "Unknown error code: $last_exit_code" ;;
+                  255) echo "SSH connection error - network/auth issue" >&2 ;;
+                  1) echo "General SSH error" >&2 ;;
+                  *) echo "Unknown error code: $last_exit_code" >&2 ;;
                 esac
 
                 if [ $attempt -lt $max_attempts ]; then
-                  echo "⏳ Waiting ${delay}s before SSH retry..."
+                  echo "⏳ Waiting ${delay}s before SSH retry..." >&2
                   sleep $delay
                 fi
                 attempt=$((attempt + 1))
               fi
             done
 
-            echo "💥 SSH command failed after $max_attempts attempts (final exit code: $last_exit_code)"
+            echo "💥 SSH command failed after $max_attempts attempts (final exit code: $last_exit_code)" >&2
             return $last_exit_code
           }
           EOF