fix(memory): add .unref() to housekeeping timers to prevent process hang

[webdevpraveen]

Adds .unref() to two setInterval timers that prevent the Node.js process from exiting cleanly.

Problem

CacheManager.startCleanupTimer() and SqlJsBackend 's auto-persist timer both call setInterval() without .unref(), keeping the event loop alive indefinitely. This means any CLI command that touches memory (memory store, memory search, etc.) hangs after completing its work and never exits - users have to Ctrl+C to kill it.

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Fix

Added .unref() after each setInterval call, guarded by an existence check for environments where .unref() may not be available:

• v3/@claude-flow/memory/src/cache-manager.ts - cleanup timer (line ~397)
• v3/@claude-flow/memory/src/sqljs-backend.ts - persist timer (line ~136)

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Why this is safe

• These are best-effort housekeeping timers, not critical work. The shutdown() methods already handle final cleanup/persist explicitly.
• Other timers in the same package (agentdb-backend.js:142, auto-memory-bridge.js:534) already call .unref() correctly - this just brings the remaining two in line with that pattern.
• Zero behavioral change when the process is still running; only affects exit behavior.

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Fixes Issue #1256