Derive idle suspension from the checkpoint declaration

[SaladDay]

Idle suspension is now one Core policy: 300 seconds idle and 86400 seconds snapshot retention. Core applies it to every Provider that declares checkpoint support. Before this change it was a microsandbox constant in the Provider registry, copied into two deployment columns and read back.

• Registry and registration: the registry no longer carries IdleSeconds/RetentionSeconds, and sandbox.Description no longer copies them. Registration keeps only the declared rule "checkpoint support ⇒ nodes mode".
• Migration 000094: drops runtime_deployment.idle_seconds/retention_seconds and rewrites runtime_deployment_setup_check without them. Down restores the old columns and values.
• Service: deployment.Service derives View.Suspension and Setup.Suspension from the checkpoint declaration in one helper.
• API: suspension keeps its shape. Its description now names the declaration rule, not a vendor.
• Docs: docs/sandbox-provider.md (en and zh) states the nodes-only rule and owns the values. sandbox-deployment.md links to them.

Production behaviour is unchanged.

Checks run locally:

• go build and go vet;
• sqlc and OpenAPI regeneration, which produced only the expected diff;
• the dependent packages with the DB (915 tests), the whole execution package and the whole tests/integration package (370 tests);
• the migration up, down and up again on a scratch DB with a configured microsandbox deployment;
• the names, link and translation checks.

This is architecture alignment T5c (decision D2). It got no blind review, because it is a small deletion lane with no behaviour change.

Generated with Claude Code

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