agent host: one representation and one cascade rule for customization enablement

Follow-up cleanup from the scoped-enablement work (#330566, #330715).

## Problem

There are three representations of "is this on?" in session state:

- `DirectoryCustomization.enabled: boolean` (`state.ts:861`)
- `ChildCustomizationBase.enabled?: boolean` (`state.ts:894`)
- `enablement?: CustomizationEnablement[]` on `PluginCustomization` and `McpServerCustomization` (`state.ts:804`, `1057`)

The tell that this is one representation too many is that the protocol doc comment has to *teach a rule you must memorize*:

> the **effective** enabled state of a plugin child is the plugin's derived enabled value and `(child.enabled ?? true)`, so a disabled plugin disables every child regardless of each child's own flag. A directory child instead uses the directory's `enabled` value and its own flag.

Two different cascade rules depending on container type, neither enforced anywhere.

Predictably, consumers hand-roll the cascade. The same expression appears in at least three places across two files:

```ts
enabled: isCustomizationEnabled(server) && (!plugin || isCustomizationEnabled(plugin))
```

- `src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostCustomizationService.ts:191`
- `src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostCustomizationService.ts:218`
- `src/vs/sessions/contrib/providers/agentHost/browser/baseAgentHostSessionsProvider.ts:3691`

There are ~32 enablement-derivation sites in total across the agent host and its consumers. Any one of them getting the cascade subtly wrong is a silently-wrong enabled state in the UI or, worse, at the SDK boundary.

## Suggestions

**Minimum viable:** export a single `isEffectivelyEnabled(child, container)` in `common/customizationEnablement.ts` and use it everywhere the cascade is currently re-derived. Cheap, no protocol change, removes the duplication.

**Better, if a protocol change is acceptable:** collapse to one representation so directories and plugins cascade identically and the doc comment no longer needs to describe two rules. Note the protocol lives in [microsoft/agent-host-protocol](https://github.com/microsoft/agent-host-protocol) (`types/` is canonical; vscode's `common/state/protocol/` mirrors it), so this needs a change there first.