feat(remote): add auth --store for a persistent control-plane credential

[outofcoffee]

spinloop remote auth --store keeps a long-lived access key for the control plane's own IAM user in the machine's OS keystore, so day-to-day remote control and fleet operations keep working between SSO log-ins.

Summary

• The control plane stack creates an IAM user (cloud-vm-llm-remote-cli) with a stack-owned managed policy scoped to day-to-day control: invoke the control URLs, read instance logs, discover the stack, price an instance, and manage the user's own access keys
• New spinloop remote auth subcommand: report the stored credential, --store (creates and verifies a new key, or rotates with the stored key alone), --clear (removes the entry and deletes the key on the AWS side)
• Credential resolution is per region: explicit environment credentials or AWS_PROFILE first, then the stored key, then the standard chain; fleet operations on remote environments sign through the same resolution
• The key is held in the OS keystore (Keychain, Credential Manager, Secret Service), or in an owner-only file under the config directory where no keystore is reachable; SPINLOOP_REMOTE_KEYSTORE=file selects the file store
• Store verification retries InvalidClientTokenId on an exponential backoff — a freshly issued key takes seconds to become resolvable
• Specs: new remote-auth capability, endpoint-provisioning and remote-endpoint updated, change archived as 2026-09-08-persistent-aws-creds

Implementation details

• The user's policy is a managed policy rather than an inline one: IAM caps a user's aggregate inline policies at 2,048 characters and the seven control functions' invoke-URL grants do not fit in it (the first deploy failed with ServiceLimitExceeded); the managed limit is 6,144
• The self-service iam ARN in the policy is built from pseudo parameters, because the policy attaches to the user it names and a direct reference would be a dependency cycle
• Control planes deployed before this change have no control-plane user; auth --store against one fails naming spinloop remote bootstrap, which is safe to re-run

Closes #172