[SaladDay]

Summary

The pinned Agents API has no Environment delete. Today, an idle suspended standalone Environment with no Session attached holds its node's retained slot until snapshot retention ends (24 h). With more prewarms than max_retained, the node blocks new placement for that whole time.

This change makes such an Environment eligible for capacity-pressure release. When a node's retained slots are full and waiting placement is blocked by them, pressure can release the node's oldest idle suspended allocation of an unattached standalone Environment. The Environment then expires exactly as at its deadline: status expired, workspace and placement released.

• New query IsUnattachedRuntimeEnvironment: the Environment is active and no live Session is attached.
• AllocationTx.CanReleaseCheckpoint is CanRetainEnvironment or unattached. It is checked under the same Environment lock as before.
• It is used by ExpireCheckpointForDemand, by the full-node suspension gate and by the oldest-peer scan.
• Cleanup, revoke, lost compute and demand qualification keep CanRetainEnvironment.
• Docs (en/zh): the cold-replacement rule in docs/sandbox-provider.md, the lifecycle without a Session in environments.md, and a ledger entry in index.md.

Verification

• go vet passes on the deployment and deploymentpg packages, and go test ./internal/deployment/ passes.
• The deploymentpg tests for pressure, cleanup and expiry pass against Postgres.
• Two new tests: an unattached Environment expires under pressure; a full node suspends an unattached owner. Both fail with the unattached branch removed.
• check-translations passes.
• Independent blind review: no blocking findings. Its five wording and redundancy nits are fixed.

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