fix(e2e): retain local dev output on failure

[Hweinstock]

Summary

• Preserve all stdout and stderr emitted by project dev in one output buffer.
• Keep the existing runtime readiness parsing and failure flow unchanged.

Spec

Failed Linux E2E job: https://github.com/aws/agentcore-cli/actions/runs/35901051494/job/107317710146

Reproduction

1. Run the local invocation E2E tests.
2. Make a local runtime fail to start or omit its readiness announcement.
3. The failure now includes the complete project dev output emitted before the failure.

Verification

• bun install --frozen-lockfile
• bun run format:check
• bun run typecheck
• bun run lint:check
• bun run build
• bun run secrets:check
• bun test src/io/exec.test.ts — 25 passed with normal child-process permissions
• Full E2E was not run locally because it requires the project’s AWS/container environment.
• The unrestricted full suite was attempted; the remaining failure was in existing subprocess timing behavior outside this E2E-only change.