refactor(executor): extract provider-neutral tool-use loop (Phase 1)

Splits LocalRunner's OpenAI-specific agentic loop into reusable, provider-
agnostic pieces so later phases can add native Anthropic/OpenAI/Google/Groq/
OpenRouter adapters without duplicating the control flow:

- internal/provider: neutral Provider/ChatRequest/ChatResponse types, plus
  an openaicompat adapter wrapping the existing internal/llm.Client unchanged
- internal/sandbox: Sandbox interface + HostSandbox (git clone/push/cleanup,
  read_file/write_file/run_bash/glob), lifted verbatim from local.go/localtools.go
- internal/agentloop: the extracted tool-use loop (request/response/tool-
  dispatch/loop, ask_user blocking, stream-json envelope, summary fallback)
- internal/agentchannel: AgentChannel/SubtaskSpec/BlockedError/ErrAgentBlocked
  moved out of internal/executor so agentloop can use them without an import
  cycle; internal/executor re-exports via type aliases, so no call site changes
- internal/executor/nativerunner.go: NativeRunner replaces LocalRunner,
  wiring agentloop.Loop + openaicompat + HostSandbox together
- config.Providers map[string]ProviderConfig added (unused until Phase 2+)

Zero intended behavior change: go test -race ./... passes across all
packages, and end-to-end stream-json/summary/changestats output was verified
byte-compatible against a fake OpenAI-compatible server. Adds test coverage
for sandbox tool-dispatch (git clone/push, read/write/bash/glob) that
LocalRunner never had.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V1moSNCJRcP6kykA4tyUSs

1 files changed, 72 insertions, 0 deletions

diff --git a/internal/provider/provider.go b/internal/provider/provider.go
new file mode 100644
index 0000000..fd1022d
--- /dev/null
+++ b/internal/provider/provider.go
@@ -0,0 +1,72 @@
+// Package provider defines a provider-neutral chat/tool-use interface. It is
+// independent of any one wire format (OpenAI-compatible, Anthropic Messages,
+// Gemini, etc.) so that internal/agentloop's tool-use control flow can drive
+// any backend that implements Provider.
+//
+// Phase 1 ships exactly one implementation, internal/provider/openaicompat,
+// which adapts the existing internal/llm.Client. Later phases add native
+// Anthropic/OpenAI/Google/Groq/OpenRouter adapters without touching agentloop.
+package provider
+
+import "context"
+
+// Message is one turn in a chat conversation, in provider-neutral shape.
+type Message struct {
+	Role        string // "system" | "user" | "assistant" | "tool"
+	Text        string
+	ToolCalls   []ToolCall   // set on assistant turns that invoke tools
+	ToolResults []ToolResult // set on tool-result turns
+}
+
+// ToolCall is a single tool invocation requested by the model.
+type ToolCall struct {
+	ID       string
+	Name     string
+	ArgsJSON string
+}
+
+// ToolResult is the outcome of executing a ToolCall, fed back to the model.
+type ToolResult struct {
+	ToolCallID string
+	Name       string
+	Content    string
+	IsError    bool
+}
+
+// ToolSpec declares a tool the model may call.
+type ToolSpec struct {
+	Name                 string
+	Description          string
+	ParametersJSONSchema map[string]any
+}
+
+// ChatRequest captures the parameters of a single chat completion call.
+type ChatRequest struct {
+	Model       string
+	System      string
+	Messages    []Message
+	Tools       []ToolSpec
+	Temperature *float64
+	MaxTokens   int
+}
+
+// Usage reports token accounting and (when known) cost for a single call.
+type Usage struct {
+	InputTokens  int
+	OutputTokens int
+	CostUSD      float64
+}
+
+// ChatResponse is the aggregated result of a chat completion.
+type ChatResponse struct {
+	Text       string
+	ToolCalls  []ToolCall
+	StopReason string
+	Usage      Usage
+}
+
+// Provider is a chat/tool-use backend: one per LLM vendor/wire-format.
+type Provider interface {
+	Name() string
+	Chat(ctx context.Context, req ChatRequest) (*ChatResponse, error)
+}