fix(sandbox): map E2B execution results

[EmersonBraun]

What changed

• model the optional error returned by E2B Code Interpreter executions
• map successful executions to exit code 0
• map cell errors to exit code 1 and include their traceback in stderr
• add regression coverage for successful and failed executions
• include a patch changeset for @agentskit/sandbox

Why

E2B Code Interpreter does not expose an exitCode on its execution result. The previous adapter read that missing field, so callers could receive an undefined exit status and lose useful failure details.

Impact

Consumers of the E2B sandbox backend now receive the documented numeric exit code contract and actionable traceback output for cell failures.

Validation

• pnpm --filter @agentskit/sandbox test — 131 tests passed
• pnpm --filter @agentskit/sandbox lint
• pnpm --filter @agentskit/sandbox build
• pnpm --filter @agentskit/sandbox test:coverage — 90.09% statements, 92.61% lines
• pnpm docs:bridge:gate
• repository pre-push quality gates and full build
• live E2B proof: JavaScript success (exit 0), Python success (exit 0), Python ZeroDivisionError (exit 1 with traceback), and sandbox disposal