v13.21.0

Installer release. Fixes the outage that made 13.20.0 uninstallable, and reworks the account/provider flow.

## Installs no longer hard-fail when cmem.ai is unreachable

13.20.0 required browser OAuth before the provider choice, unconditionally. When the server endpoint it depended on turned out not to be deployed, every install died on "Could not start OAuth login."

`--provider claude` now skips login entirely. It configures memory against your own Anthropic plan and never contacts cmem.ai, so there is no account question for login to answer.

Keyed on the explicit flag, not on reachability: falling back to a local install whenever cmem.ai happened to be down would silently change what you get. `gemini` and `openrouter` stay gated, because openrouter is the transport for the cmem gateway.

## The trial length is no longer pinned to 7 days

The installer used to require the checkout URL to say exactly `trial=7`, so the server could not change its own offer without breaking every published installer — and the rejection surfaced as "Could not start OAuth login", naming nothing. The URL shape is still validated strictly; the number is now the server's to choose.

## The provider screen is two lines

    Select Provider:
    ================
    [ ] CMEM Pro (30 Day Free Trial: Tokens for Observations + Real-Time Cloud
        Sync for Claude.ai, ChatGPT.com, anything that accepts an MCP Connector)
    [ ] Use your Anthropic Max Plan (no cloud sync, uses tokens for observations)

The nine-bullet benefits note that printed above it is gone.

## The billing acknowledgement moved to checkout

It was a terminal prompt asking you to confirm charge terms before you could see what you were agreeing to. It is a term of the charge, so it now appears on the screen that takes the card, above the pay button.

## Login is only about logging in

- A server-reported checkout stage during login no longer renders "Waiting for CMEM Pro setup in the browser...".
- The login hand-off prints the URL, then waits: `Continue setup in browser... (hit return to open automatically)`. The URL comes first, so headless and SSH sessions are never blocked.
- The post-login browser page now says "Close this window and go back to your terminal" instead of linking to the dashboard, which abandoned an install still waiting on that round-trip.

Fixed: the checkout hand-off briefly also waited for Return, which stalled the install outright. It opens directly again.

## Next Steps is shorter

Dropped the `CLAUDE_MEM_WELCOME_HINT_ENABLED` opt-out, the uninstall warning, and the A/B framing that presented "just start working" as a decision.