add content blocks and OpenAI Responses provider

[writeameer]

Summary

• replace string-only messages with typed content blocks for text, images, tool calls, and tool results
• keep agent.Ask(ctx, string) as the simple text path and add AskContent for rich input
• change provider responses to return message slices, expand token usage, and add optional capabilities
• move the OpenAI provider to the Responses API with text and image input support
• update xAI, Anthropic, examples, memory copying, tests, and docs for the new core shape

Validation

• GOCACHE=/private/tmp/forge-go-cache go test ./...
• _examples/hello-world: GOCACHE=/private/tmp/forge-go-cache go test ./...
• _examples/chat-console: GOCACHE=/private/tmp/forge-go-cache go test ./...
• _examples/calculator: GOCACHE=/private/tmp/forge-go-cache go test ./...

Notes

• this is intentionally a breaking API change before wider adoption
• after merge, cut a new Forge version and update hello-forge off its temporary local replace