Two bugs found while reworking #2653.
**Custom flags throw.** `SyncStaticFlag` is a switch expression with no discard arm, so any key that has no static behind it throws `SwitchExpressionException`. `[FeatureFlag mykey create <category> <desc>` crashes in `CreateOrUpdateFlag`, and once such a key is in `flags.json`, every boot's `SyncAllStaticFlags()` throws mid-iteration — caught and logged as "Failed to load feature flags", but stock flags later in dictionary order never sync. Added `_ => enabled`.
**Removing a flag turns some features on.** `RemoveFlag` hardcoded `SyncStaticFlag(flagKey, true)`. For `speedhack_detection` and `insurance` the real default is `false` / `insurance.enable`, so deleting the flag enabled the feature. It now syncs the removed flag's `DefaultEnabled`, which #2653 seeds from the static.
Verified: `dotnet build Projects/UOContent/UOContent.csproj -c Release`, 0 `CS` diagnostics (post-build copy to `Distribution/` was blocked by a running server instance).
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