fix: prevent task count line break in session-start template

grep -c returns empty on no match, causing arithmetic to break
across lines. Use ${var:-0} fallback pattern.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/repo/scripts/agent/session-start.sh

@@ -45,9 +45,12 @@ if [[ -f "$MISSION_JSON" ]] && command -v jq &>/dev/null; then
 
     # Task counts
     if [[ -f "docs/TASKS.md" ]]; then
-      total="$(grep -c '^|' "docs/TASKS.md" 2>/dev/null || echo 0)"
+      total="$(grep -c '^|' "docs/TASKS.md" 2>/dev/null || true)"
-      done_count="$(grep -ci '| done \|| completed ' "docs/TASKS.md" 2>/dev/null || echo 0)"
+      total="${total:-0}"
-      echo "  Tasks: ~$done_count done of ~$((total - 2)) total"
+      done_count="$(grep -ci '| done \|| completed ' "docs/TASKS.md" 2>/dev/null || true)"
+      done_count="${done_count:-0}"
+      approx_total=$(( total > 2 ? total - 2 : 0 ))
+      echo "  Tasks: ~${done_count} done of ~${approx_total} total"
     fi
 
     # Scratchpad