refactor: add shellcheck source directives for provider variables

- Add shellcheck source comments to all agent scripts
- Tells shellcheck where provider-exported variables are defined
- Fix 132+ SC2154 warnings across all providers

Score: 30 (Impact: 6, Confidence: 10, Risk: 2)

lambda/codex.sh

@@ -5,6 +5,7 @@ set -e
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" 2>/dev/null && pwd)"
 if [[ -f "$SCRIPT_DIR/lib/common.sh" ]]; then
     source "$SCRIPT_DIR/lib/common.sh"
+# shellcheck source=lambda/lib/common.sh
 else
     eval "$(curl -fsSL https://raw.githubusercontent.com/OpenRouterTeam/spawn/main/lambda/lib/common.sh)"
 fi