MCP `egress_grants`: list, grant and revoke an app's private destinations, with the same rules as the page #3434

Split from #3369, so an agent has the same action as the person (same words, same refusals).

Done:

• [ ] One MCP tool lists an app's asked-for private destinations and their grants, and grants or revokes one for a site admin, through the same action as the page.
• [ ] A non-admin's grant is refused with the same message the API gives.
• [ ] test/mcp.test.ts covers list, grant and the refusal; tools/list stays within its size limit.

Files: src/http/mcp.ts, test/mcp.test.ts. Do not touch src/tui/.