feat: streaming del subproceso claude-code con --output-format stream-json

Implementa la Fase 1 del issue 0036: soporte de streaming en tiempo real
para el provider claude-code.

- Tipos puros de streaming en pkg/llm/types.go: StreamEventKind,
  StreamEvent, StreamFunc (pure core, sin side effects)
- Refactor de shell/llm/claudecode.go: nuevo code path executeStreaming
  que usa cmd.StdoutPipe + bufio.Scanner para leer linea a linea
- Parser parseStreamLine que mapea eventos JSON del CLI (system, assistant,
  result) a StreamEvent del dominio
- buildClaudeArgs ahora selecciona --output-format stream-json cuando
  streaming esta habilitado y StreamFunc presente
- Campos Streaming y ShowToolProgress en ClaudeCodeCfg (config schema)
- Backward compatible: streaming=false (default) no cambia comportamiento
- 40 tests (20 existentes + 20 nuevos) pasan sin errores

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/llm/types.go

@@ -42,13 +42,14 @@ type ToolSpec struct {
 }
 
 type CompletionRequest struct {
-	Model       string
-	Messages    []Message
-	Tools       []ToolSpec
-	MaxTokens   int
-	Temperature float64
-	Stream      bool
+	Model        string
+	Messages     []Message
+	Tools        []ToolSpec
+	MaxTokens    int
+	Temperature  float64
+	Stream       bool
 	SystemPrompt string
+	StreamFunc   StreamFunc // optional: if set, streaming events are emitted during execution
 }
 
 type TokenUsage struct {
@@ -67,3 +68,34 @@ type CompletionResponse struct {
 // CompleteFunc is the single contract for LLM providers.
 // Implementations live in shell/llm/.
 type CompleteFunc func(ctx context.Context, req CompletionRequest) (CompletionResponse, error)
+
+// ── Streaming types (pure) ───────────────────────────────────────────────
+
+// StreamEventKind identifies the kind of streaming event emitted by
+// a claude-code subprocess running with --output-format stream-json.
+type StreamEventKind string
+
+const (
+	StreamInit       StreamEventKind = "init"
+	StreamToolUse    StreamEventKind = "tool_use"
+	StreamToolResult StreamEventKind = "tool_result"
+	StreamText       StreamEventKind = "text"
+	StreamResult     StreamEventKind = "result"
+	StreamError      StreamEventKind = "error"
+)
+
+// StreamEvent carries a single streaming event from the claude subprocess.
+// Fields are populated based on Kind; not all fields are valid for all kinds.
+type StreamEvent struct {
+	Kind      StreamEventKind
+	ToolName  string // tool_use: name of the tool being invoked
+	ToolInput string // tool_use: truncated input description
+	Content   string // text/result: textual content
+	IsError   bool   // result: whether the result indicates an error
+	Error     error  // error: the error that occurred
+}
+
+// StreamFunc is the callback invoked for each streaming event.
+// Implementations must be safe for concurrent use (typically not needed
+// since the streaming loop calls sequentially).
+type StreamFunc func(event StreamEvent)