feat: typed customFields generics + optional companyId

[priosshrsth]

Summary

• Typed customFields generics: createAssemblyKit / AssemblyKit now accept optional TClientCustomFields and TCompanyCustomFields generics that flow through kit.clients.* and kit.companies.*. Defaults to Record<string, unknown> so existing callers are unaffected.
• Optional companyId in tokens: client tokens no longer require companyId. Schema refine now requires only internalUserId or clientId; isClientUser and ensureIsClient updated accordingly.
• Dependency bumps: @typescript/native-preview to latest dev build; transport updated for ky 2.x prefixUrl → prefix rename. AGENTS.md regenerated by vp config post-bump.

Usage (new generic)

// Single shape applied to both clients and companies
const kit = createAssemblyKit<{ roles: string[] }>({ apiKey, workspaceId });
const c = await kit.clients.retrieve("id");
c.customFields; // { roles: string[] } | null | undefined

// Different shapes per resource
const kit = createAssemblyKit<{ roles: string[] }, { industry: string }>(opts);

Both generics are optional — passing none keeps the previous Record<string, unknown> behavior.

Test plan

• bun run check passes (lint, format, types — isolatedDeclarations enabled)
• bun run test — all 172 existing tests pass; added a positive case for clientId-only client tokens and flipped the negative case to companyId-only
• Verify in a downstream consumer that kit.clients.retrieve() infers the supplied customFields shape

🤖 Generated with Claude Code