feat(provider): add native Anthropic Messages API adapter (Phase 2)

Adds internal/provider/anthropic, the first genuinely new provider.Provider
implementation on top of Phase 1's provider-neutral tool-use loop, alongside
(not replacing) the existing Docker/CLI-subprocess ContainerRunner path for
the "claude" agent type:

- internal/provider/anthropic: translates the neutral ChatRequest/ChatResponse
  shape to/from Anthropic's Messages API content-block format (system as a
  top-level field, tool_use/tool_result blocks, no "tool" role -- tool
  results become user-role messages), with a per-model-prefix pricing table
  for CostUSD
- internal/retry: IsRateLimitError additively extended to recognize
  Anthropic's rate_limit_error/overloaded_error/529 shapes
- internal/config: RunnersConfig.Anthropic/AnthropicEnabled() gate
- internal/cli/serve.go, run.go: register runners["anthropic"] as a
  NativeRunner when [providers.anthropic].api_key is set and enabled --
  tracked as a distinct executions.agent="anthropic" budget bucket, separate
  from the CLI-subprocess "claude" runner even though both bill the same
  Anthropic account

go build/vet/test -race all pass. No live Anthropic API key is available in
this environment, so verification is via fake-httptest-server adapter tests
(12 cases, incl. multi-turn tool_result round-trip and rate-limit error
matching) plus a Pool/NativeRunner routing test proving agent.type:
"anthropic" actually reaches the new provider. Live end-to-end verification
against the real API is a follow-up once a key is configured.

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

1 files changed, 87 insertions, 0 deletions

diff --git a/internal/executor/anthropic_routing_test.go b/internal/executor/anthropic_routing_test.go
new file mode 100644
index 0000000..8794dd8
--- /dev/null
+++ b/internal/executor/anthropic_routing_test.go
@@ -0,0 +1,87 @@
+package executor
+
+import (
+	"context"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"sync/atomic"
+	"testing"
+
+	"github.com/thepeterstone/claudomator/internal/provider/anthropic"
+)
+
+// TestPool_Submit_AnthropicAgentType_RoutesThroughAnthropicProvider is the
+// NativeRunner-level confirmation (Phase 2 verification item 5) that a task
+// with agent.type: "anthropic" is actually routed through the new
+// internal/provider/anthropic.Provider when the pool is wired with an
+// "anthropic" runner — exactly how internal/cli/serve.go and
+// internal/cli/run.go register it from cfg.Providers["anthropic"] when a
+// non-empty APIKey is configured. This exercises the full path: Pool.Submit
+// -> execute() -> NativeRunner.Run() -> agentloop.Loop.Run() ->
+// anthropic.Provider.Chat() -> HTTP POST to the (fake) Anthropic Messages
+// API endpoint.
+func TestPool_Submit_AnthropicAgentType_RoutesThroughAnthropicProvider(t *testing.T) {
+	var calls int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		atomic.AddInt32(&calls, 1)
+		if r.URL.Path != "/v1/messages" {
+			t.Errorf("unexpected path %q", r.URL.Path)
+		}
+		if r.Header.Get("x-api-key") != "test-anthropic-key" {
+			t.Errorf("wrong x-api-key: %q", r.Header.Get("x-api-key"))
+		}
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{
+			"content": [{"type": "text", "text": "## Summary\ndone"}],
+			"stop_reason": "end_turn",
+			"model": "claude-sonnet-5",
+			"usage": {"input_tokens": 12, "output_tokens": 4}
+		}`))
+	}))
+	defer srv.Close()
+
+	// Mirrors the exact construction in internal/cli/serve.go's
+	// cfg.Providers["anthropic"] wiring block: anthropic.New(apiKey,
+	// endpoint, timeout) wrapped in a NativeRunner registered under the
+	// "anthropic" runner-map key.
+	runners := map[string]Runner{
+		"anthropic": &NativeRunner{
+			Provider: anthropic.New("test-anthropic-key", srv.URL, 0),
+			Logger:   slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError})),
+			LogDir:   t.TempDir(),
+		},
+	}
+
+	store := testStore(t)
+	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError}))
+	pool := NewPool(2, runners, store, logger)
+
+	tk := makeTask("anthropic-routing-1")
+	tk.Agent.Type = "anthropic"
+	tk.Agent.Model = "claude-sonnet-5"
+	tk.Agent.SkipPlanning = true
+	if err := store.CreateTask(tk); err != nil {
+		t.Fatalf("CreateTask: %v", err)
+	}
+
+	if err := pool.Submit(context.Background(), tk); err != nil {
+		t.Fatalf("Submit: %v", err)
+	}
+
+	result := <-pool.Results()
+	if result.Err != nil {
+		t.Fatalf("expected no error, got: %v", result.Err)
+	}
+	if got := atomic.LoadInt32(&calls); got != 1 {
+		t.Fatalf("expected exactly 1 call to the fake Anthropic server, got %d", got)
+	}
+	if result.Execution.Agent != "anthropic" {
+		t.Errorf("execution.Agent: want %q, got %q", "anthropic", result.Execution.Agent)
+	}
+	if result.Execution.TokensIn != 12 || result.Execution.TokensOut != 4 {
+		t.Errorf("tokens should come from the anthropic provider's usage: got in=%d out=%d",
+			result.Execution.TokensIn, result.Execution.TokensOut)
+	}
+}