feat(integrations): add StackAdapt MCP entry

[sdhilip200]

Summary

• add a Cabinet-maintained read-only StackAdapt MCP server under mcps/mcp-stackadapt
• wire StackAdapt into MCP_CATALOG as a token-backed stdio CatalogEntry
• add the visible integrations hub card/icon and launch it in the current preview gate

Architecture notes

• follows the CatalogEntry path requested in feat: add Stripe connector #35 instead of adding server/connectors/* code
• defers to the official Meta Ads MCP already on main; this PR no longer adds a custom Meta Ads MCP server
• stores the StackAdapt token in .cabinet.env and injects it into the MCP server process at runtime
• exposes read-only StackAdapt reporting tools; GraphQL mutations and subscriptions are rejected before a request is sent

Testing

• npm run typecheck --prefix mcps/mcp-stackadapt
• npm run smoke --prefix mcps/mcp-stackadapt
• npx tsc --noEmit
• npm run lint
• npm run build

Refs #106

[sdhilip200]

Superseded by the latest branch update: #231 now defers to the official Meta Ads MCP entry on main and no longer includes a custom Meta Ads MCP server. Current scope is StackAdapt only.

[sdhilip200]

Updated this PR to match the current Meta Ads direction from #30, and narrowed the PR title/body to StackAdapt.

The branch now defers to the official Meta Ads MCP entry already on main and removes the custom Meta Ads MCP server from this PR. The remaining scope is StackAdapt only: a Cabinet-maintained read-only MCP server plus the CatalogEntry and integrations hub wiring.

Revalidated with:

• npm run typecheck --prefix mcps/mcp-stackadapt
• npm run smoke --prefix mcps/mcp-stackadapt
• npx tsc --noEmit
• npm run lint
• npm run build

GitHub currently reports the branch as mergeable; CI is running on the latest force-push.