fix: auto-correct prefix conflicts in create-new-feature.sh

When --number is passed with a prefix that already exists in the specs/
directory (e.g., an AI agent passes --number 1 but specs/001-* already
exists), the script now auto-corrects to the next globally available
number instead of silently creating a duplicate.

This prevents duplicate spec directory prefixes that cause errors in
downstream tooling (e.g., check-prerequisites.sh emitting
"ERROR: Multiple spec directories found with prefix '001'").

The --number flag becomes a validated suggestion: if it conflicts,
the script recalculates from the true global maximum across all spec
directories and git branches, then warns on stderr:

  ⚠️  Requested number 001 conflicts with existing spec directory.
      Auto-corrected to 030.

The workflow never fails due to numbering — it self-heals.

Author: daniel-graham

scripts/bash/create-new-feature.sh

@@ -255,6 +255,30 @@ fi
 
 # Force base-10 interpretation to prevent octal conversion (e.g., 010 → 8 in octal, but should be 10 in decimal)
 FEATURE_NUM=$(printf "%03d" "$((10#$BRANCH_NUMBER))")
+
+# ── Guardrail: auto-correct if the chosen prefix already exists ──────────────
+# If a specs directory with this numeric prefix already exists, the requested
+# number is stale or wrong (e.g., an agent passed --number with a conflicting
+# value). Auto-correct to the global max + 1 so the workflow never fails due
+# to numbering collisions.
+if compgen -G "$SPECS_DIR/${FEATURE_NUM}-*" > /dev/null 2>&1; then
+    REQUESTED_NUM="$FEATURE_NUM"
+    # Recalculate from the true global max across specs + branches
+    if [ "$HAS_GIT" = true ]; then
+        GLOBAL_MAX_BRANCH=$(get_highest_from_branches)
+    else
+        GLOBAL_MAX_BRANCH=0
+    fi
+    GLOBAL_MAX_SPEC=$(get_highest_from_specs "$SPECS_DIR")
+    GLOBAL_MAX=$GLOBAL_MAX_BRANCH
+    if [ "$GLOBAL_MAX_SPEC" -gt "$GLOBAL_MAX" ]; then
+        GLOBAL_MAX=$GLOBAL_MAX_SPEC
+    fi
+    BRANCH_NUMBER=$((GLOBAL_MAX + 1))
+    FEATURE_NUM=$(printf "%03d" "$((10#$BRANCH_NUMBER))")
+    >&2 echo "⚠️  Requested number $REQUESTED_NUM conflicts with existing spec directory. Auto-corrected to $FEATURE_NUM."
+fi
+
 BRANCH_NAME="${FEATURE_NUM}-${BRANCH_SUFFIX}"
 
 # GitHub enforces a 244-byte limit on branch names