no guide for an MSSP reading several client tenants with the CLI #45

Measured

• README.md:104-107 sums it up in one sentence: "To use another tenant, sign in again with another profile (--profile)". There is no section explaining how an MSSP reads several client tenants.
• Each credential covers a single tenant: an API key belongs to one tenant, and an OAuth session fixes the tenant on the consent screen (README.md:83-107, openapi/v1.json → info.description).

Why it matters

MSSP beta tester: manages several clients and, on day one, needs to know how to read each one, or all of them, without mixing their data.

Acceptance criteria

• The README has a "Several tenants (MSSP)" section covering: signing in once with an MSSP credential, listing the authorized client tenants, selecting one client or all of them on read commands, checking which client a command reads, and revoking access.
• It does not recommend one profile per client as the way to read a portfolio.
• The walkthrough has been tested end to end against two client tenants of a test MSSP account (see sign-in of an MSSP user on a client tenant has not been tested on Linux with the published version #46).

Depends on

#54 (portfolio view and client selection), #46 (MSSP sign-in test). Later improvement: #48 (have whoami show the tenant).