Add a third WorkloadManager implementation that creates sandboxes via
the compute gateway HTTP API (POST /api/sandboxes). Uses native fetch
with no new dependencies. Enabled by setting COMPUTE_GATEWAY_URL, which
takes priority over Kubernetes and Docker providers.

The fetch() call had no timeout, causing infinite hangs when the gateway
accepted requests but never returned responses. Adds AbortSignal.timeout
(30s) and consolidates all logging into a single structured event per
create() call with timing, status, and error context.

Emit a single canonical log line in a finally block instead of scattered
log calls at each early return. Adds business context (envId, envType,
orgId, projectId, deploymentVersion, machine) and instanceName to the
event. Always emits at info level with ok=true/false for queryability.

Pass business context (runId, envId, orgId, projectId, machine, etc.)
as metadata on CreateSandboxRequest instead of relying on env vars.
This enables wide event logging in the compute stack without parsing
env or leaking secrets.

Passes machine preset cpu and memory as top-level fields on the
CreateSandboxRequest so the compute stack can use them for admission
control and resource allocation.

Thread timing context from queue consumer through to the compute
workload manager's wide event:
- dequeueResponseMs: platform dequeue HTTP round-trip
- pollingIntervalMs: which polling interval was active (idle vs active)
- warmStartCheckMs: warm start check duration
All fields are optional to avoid breaking existing consumers.

...-manager