refactor(site): convert OrganizationAutocomplete to a fully controlled component

[johnstcn]

Context

OrganizationAutocomplete currently manages its own selected state internally and exposes an onChange callback. PR coder/coder#23906 added an organizationId prop with render-time derivation to support controlled pre-selection, but the component is still a hybrid controlled/uncontrolled component.

Every other autocomplete/select component in the codebase (Autocomplete, UserAutocomplete, SelectFilter, MultiUserSelect) is fully controlled: the parent owns the value, and the component just renders and calls onChange.

Proposal

Refactor OrganizationAutocomplete to match the fully controlled pattern:

• Accept value: Organization | null and onChange from the parent
• Remove internal selected state entirely
• Move the auto-select-single-org logic to callers (or a shared hook)
• Update both callers (CreateUserForm, CreateTemplateForm) and all stories

This aligns the component with established codebase patterns and eliminates the remaining hybrid state complexity.

References

• PR github.com/feat(site): add organizationId prop to OrganizationAutocomplete coder#23906 (adds organizationId prop with render-time derivation)