Omit empty releaseStrategy from UpdateInstanceInput

[coryodaniel]

Summary

mdClient.Instances.Update(ctx, id, instances.UpdateInput{Version: "latest+dev"}) fails before reaching the resolver:

input:3: Argument "input" has invalid value $input.
In field "releaseStrategy": Expected type "ReleaseStrategy", found "".

The V2 schema's UpdateInstanceInput marks releaseStrategy as deprecated and nullable. Clients that followed the migration to the +dev suffix on version shouldn't have to send the field anymore. In practice they couldn't, because the generated gen.UpdateInstanceInput emitted "releaseStrategy": "" whenever a caller left it unset, and Absinthe rejects the empty string against the ReleaseStrategy enum.

So every deprecation-following caller of Instances.Update was broken — every mass instance version <id>@latest+dev invocation, every preview command's per-instance version override, etc.

Same shape as #24 (CopyInstanceInput.Message), different field; the previous fix was a schema removal but this field is still in the schema (just deprecated), so this PR uses a genqlient directive instead.

Change

One genqlient directive on the operation:

# @genqlient(for: "UpdateInstanceInput.releaseStrategy", omitempty: true, pointer: true)
mutation UpdateInstance(
  $organizationId: ID!,
  $id: ID!,
  $input: UpdateInstanceInput!
) { ... }

Regenerated zz_generated.go. The field's Go shape moves from typed string to *ReleaseStrategy with omitempty — nil pointer ⇒ field absent on the wire, matching the deprecated + nullable schema contract.

Public instances.UpdateInput already only exposes Version, so nothing in the wrapper changes — the gen-level mapping gen.UpdateInstanceInput{Version: input.Version} keeps working.

Test plan

• go build ./...
• go test ./... — all packages green
• CI

Fixes #25. Unblocks massdriver-cloud/mass#236 and #238.