Avoid forced process exit on shutdown

[x-Spartacus]

Closes #114

• Remove process.exit() calls from the SIGINT/SIGTERM shutdown handler
• Preserve cleanup and error logging
• Clear pending idle auto-capture timeout during cleanup
• Set process.exitCode = 1 on cleanup failure without forcing host process teardown
• Add regression coverage so process.exit() is not reintroduced

Tested:

• npx bun test tests/plugin-shutdown.test.ts tests/plugin-loader-contract.test.ts
• npx bun run typecheck
• npx bun run build
• npx bun test