fix: allow persistence-safe shutdown budget

[Xipong]

Summary

• replace the fixed 10-second forced-exit watchdog with centralized CAMOFOX_SHUTDOWN_TIMEOUT_MS configuration
• use a persistence-safe 45-second default
• reject zero, negative, non-integer, infinite, unsafe-integer, and above-Node-timer-limit values by falling back to the default
• forward the setting through the existing explicit child-process environment whitelist

Motivation

Graceful shutdown awaits server:shutdown listeners before closing browser contexts. The persistence plugin uses that window to checkpoint storage state, optionally including IndexedDB. With multiple active sessions or larger IndexedDB state, the existing 10-second watchdog can terminate the process while those awaited checkpoints are still running.

The watchdog remains bounded and armed before shutdown work begins; only its validated budget becomes configurable.

Verification

• npm test -- --runInBand tests/unit/config.test.js tests/unit/shutdownBudget.test.js tests/unit/serverShutdownEvent.test.js plugins/persistence/plugin.test.js plugins/persistence/persistence.test.js
• npm run build
• npm run test:mcp
• CI-equivalent unit + plugin suite: 59 suites / 786 tests passed
• node --check server.js

The new regression tests were added before the implementation and failed while gracefulShutdown() still used the fixed 10-second value.

Scope

This does not change checkpoint ordering, make persistence concurrent, enable IndexedDB by default, or alter normal runtime behavior. The only user-visible difference is the bounded wait during shutdown.