MCP `subscriptions`: the Claude accounts the crew works on and where each is used, never a token, for a forge admin #5418

For: the owner can see from a harness which subscription pays for which repository, the way /settings/subscriptions (src/web/pages/subscriptions.ts) shows it.

Done:

• [ ] A read-only tool subscriptions answers each subscription from listSubscriptions: name, the scopes it pays for (scopesOf), its budgets (budgetsOf) and its state. No token, key or credential field is ever in the answer. Admin only, refused in the page's words.
• [ ] test/mcp-subscriptions.test.ts covers three cases: an admin sees a subscription added with a token, and the answer's JSON does not contain that token anywhere; scopes and budgets read back; a non-admin is refused. Each fails without the tool.
• [ ] One row in docs/MCP.md.
• [ ] Before pushing, run the new test, test/mcp.test.ts, test/mcp-docs.test.ts and test/mcp-size.test.ts. A red test elsewhere that main also has is named in the pull, not a reason to stop.

Risk: test/mcp-size.test.ts caps tools/list. Keep the description to one short sentence and the tool to no arguments.