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, 25 insertions, 0 deletions

diff --git a/internal/sandbox/sandbox.go b/internal/sandbox/sandbox.go
new file mode 100644
index 0000000..4275475
--- /dev/null
+++ b/internal/sandbox/sandbox.go
@@ -0,0 +1,25 @@
+// Package sandbox defines the filesystem/shell/git surface an agent loop
+// executes tool calls against. Sandbox is intentionally narrow — just the
+// operations internal/agentloop's read_file/write_file/run_bash/glob tools
+// and its git clone/push/cleanup lifecycle need.
+//
+// Phase 1 ships exactly one implementation, HostSandbox, which lifts the
+// LocalRunner's existing behavior (host git clone into a temp dir, plain
+// os.Exec-backed file/bash tools) verbatim. A DockerSandbox (isolation
+// improvements, new safety checks) is explicitly out of scope for this phase.
+package sandbox
+
+import "context"
+
+// Sandbox is the environment a single task execution runs its tool calls
+// against.
+type Sandbox interface {
+	ReadFile(ctx context.Context, path string) (string, error)
+	WriteFile(ctx context.Context, path, content string) error
+	RunBash(ctx context.Context, command string) (stdout, stderr string, exitCode int, err error)
+	Glob(ctx context.Context, pattern string) ([]string, error)
+	GitClone(ctx context.Context, remote, branch string) error
+	GitPush(ctx context.Context, branch string) error
+	WorkDir() string
+	Cleanup(ctx context.Context) error
+}