Add go-llm v2: redesigned API for simpler LLM abstraction

v2 is a new Go module (v2/) with a dramatically simpler API:
- Unified Message type (no more Input marker interface)
- Define[T] for ergonomic tool creation with standard context.Context
- Chat session with automatic tool-call loop (agent loop)
- Streaming via pull-based StreamReader
- MCP one-call connect (MCPStdioServer, MCPHTTPServer, MCPSSEServer)
- Middleware support (logging, retry, timeout, usage tracking)
- Decoupled JSON Schema (map[string]any, no provider coupling)
- Sample tools: WebSearch, Browser, Exec, ReadFile, WriteFile, HTTP
- Providers: OpenAI, Anthropic, Google (all with streaming)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/CLAUDE.md

@@ -0,0 +1,31 @@
+# CLAUDE.md for go-llm v2
+
+## Build and Test Commands
+- Build project: `cd v2 && go build ./...`
+- Run all tests: `cd v2 && go test ./...`
+- Run specific test: `cd v2 && go test -v -run <TestName> ./...`
+- Tidy dependencies: `cd v2 && go mod tidy`
+- Vet: `cd v2 && go vet ./...`
+
+## Code Style Guidelines
+- **Indentation**: Standard Go tabs
+- **Naming**: `camelCase` for unexported, `PascalCase` for exported
+- **Error Handling**: Always check and handle errors immediately. Wrap with `fmt.Errorf("%w: ...", err)`
+- **Imports**: Standard library first, then third-party, then internal packages
+
+## Package Structure
+- Root package `llm` — public API (Client, Model, Chat, ToolBox, Message types)
+- `provider/` — Provider interface that backends implement
+- `openai/`, `anthropic/`, `google/` — Provider implementations
+- `tools/` — Ready-to-use sample tools (WebSearch, Browser, Exec, ReadFile, WriteFile, HTTP)
+- `internal/schema/` — JSON Schema generation from Go structs
+- `internal/imageutil/` — Image compression utilities
+
+## Key Design Decisions
+1. Unified `Message` type instead of marker interfaces
+2. `map[string]any` JSON Schema (no provider coupling)
+3. Tool functions return `(string, error)`, use standard `context.Context`
+4. `Chat.Send()` auto-loops tool calls; `Chat.SendRaw()` for manual control
+5. MCP one-call connect: `MCPStdioServer(ctx, cmd, args...)`
+6. Streaming via pull-based `StreamReader.Next()`
+7. Middleware for logging, retry, timeout, usage tracking