fix(shared): recheck debug logging flag

[re-marked]

Summary

• re-read DEBUG each time logForDebugging runs instead of snapshotting at module load
• support trimmed truthy DEBUG values through the shared env helper
• add disableDebugLogging() and export it for runtime/test cleanup
• document the runtime debug logging contract

Closes #70.

Verification

• pnpm test -- packages/shared/src/debug.test.ts
• pnpm --filter @yokai-tui/shared typecheck
• pnpm --filter @yokai-tui/shared lint
• pnpm --filter @yokai-tui/shared build