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/errors.go

@@ -0,0 +1,17 @@
+package llm
+
+import "errors"
+
+var (
+	// ErrNoToolsConfigured is returned when the model requests tool calls but no tools are available.
+	ErrNoToolsConfigured = errors.New("model requested tool calls but no tools configured")
+
+	// ErrToolNotFound is returned when a requested tool is not in the toolbox.
+	ErrToolNotFound = errors.New("tool not found")
+
+	// ErrNotConnected is returned when trying to use an MCP server that isn't connected.
+	ErrNotConnected = errors.New("MCP server not connected")
+
+	// ErrStreamClosed is returned when trying to read from a closed stream.
+	ErrStreamClosed = errors.New("stream closed")
+)