support standard schema / non-zod schemas for host tools

[LeonardoTrapani]

host tool definitions are currently zod-specific.
today, inputSchema is typed as ZodType, converted with zodToJsonSchema, and validated with zod-specific parsing. that makes the host tool api harder to use with other schema libraries such as effect schema, valibot, arktype, and other libraries that implement Standard Schema.
relevant places:

• packages/core/src/host-tools.ts
• packages/core/src/host-tools-zod.ts
• packages/core/src/agent-os.ts

proposal

make host tool schemas schema-library-agnostic by supporting Standard Schema-compatible validators, instead of requiring zod specifically.
ideally the api would support:

• Standard Schema validation for runtime input parsing
• JSON Schema generation when available via Standard JSON Schema
• raw JSON Schema + custom validation as an escape hatch
• backwards compatibility for existing zod-based tool definitions
  other libraries like trpc, hono etc... do a similar thing to be able to for example define route definitions with any schema library https://hono.dev/docs/guides/validation
  this would:
• remove unnecessary vendor lock-in to zod
• make host tools easier to use with libraries like arktype, effect schema and other modern schema libraries
• align the api better with the broader TypeScript ecosystem, where many tools now accept Standard Schema-compatible inputs
• preserve the current zod experience while broadening compatibility

i'd be happy to implement this myself if the direction sounds good.