[Story] Per-server onlyAllow field in MCP config #571

Context

Some MCP servers expose 100+ tools, but a user typically wants only a handful. Today the only way to limit them is `disabledTools`, which becomes an unwieldy 97-entry denylist when you want to enable just 3 tools.

Closes: RooCodeInc/Roo-Code#11971.

This is also the first scoping primitive that the dynamic tool loading work depends on — it shrinks the candidate pool the ranker has to search.

Developer Notes

• Add `onlyAllow?: string[]` to the per-server MCP config schema (`packages/types/src/mcp.ts`).
• When `onlyAllow` is set and non-empty: enable only the listed tools (`enabledForPrompt: true`); disable all other tools on that server. Takes precedence over `disabledTools` on the same server.
• When unset or empty: existing `disabledTools` behavior is unchanged.
• Apply the filter in `McpHub.fetchToolsList()` (`src/services/mcp/McpHub.ts`) where `enabledForPrompt` is set.
• Update the MCP settings UI (`webview-ui/src/components/mcp/McpView.tsx`) to surface the field.

Acceptance Criteria

• `onlyAllow` field added to MCP server config schema
• Unit test: `onlyAllow: [a, b]` with `disabledTools: [a]` → `a` is enabled (onlyAllow wins), `b` is enabled, all others disabled
• Unit test: `onlyAllow` unset → `disabledTools` behavior unchanged
• MCP settings UI exposes the field