test(01-02): add failing tests then implement session + state

- TailReadJSONL: reverse-read JSONL without loading entire file
- FindSessionForProcess: match PID to most recent JSONL via EncodePath
- DetectState: heuristic based on last message stop_reason + tool name
- ExtractPreview: extract first 3 lines of last assistant text
- All 17 tests pass (session_test.go + state_test.go)

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

state_test.go

@@ -0,0 +1,182 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+var testNow = time.Date(2026, 3, 23, 12, 0, 30, 0, time.UTC)
+
+func TestDetectState_EndTurnText(t *testing.T) {
+	msgs := []JSONLMessage{{
+		Type:      "assistant",
+		Timestamp: "2026-03-23T12:00:00Z",
+		Message: &MessagePayload{
+			Role:       "assistant",
+			Content:    []ContentBlock{{Type: "text", Text: "Done."}},
+			StopReason: "end_turn",
+		},
+	}}
+
+	state := DetectState(msgs, testNow)
+	if state != NeedsInput {
+		t.Errorf("expected NeedsInput, got %v", state)
+	}
+}
+
+func TestDetectState_ToolUseAskUser(t *testing.T) {
+	msgs := []JSONLMessage{{
+		Type:      "assistant",
+		Timestamp: "2026-03-23T12:00:00Z",
+		Message: &MessagePayload{
+			Role: "assistant",
+			Content: []ContentBlock{
+				{Type: "text", Text: "Let me ask..."},
+				{Type: "tool_use", Name: "AskUserQuestion"},
+			},
+			StopReason: "tool_use",
+		},
+	}}
+
+	state := DetectState(msgs, testNow)
+	if state != NeedsInput {
+		t.Errorf("expected NeedsInput, got %v", state)
+	}
+}
+
+func TestDetectState_ToolUseOther(t *testing.T) {
+	msgs := []JSONLMessage{{
+		Type:      "assistant",
+		Timestamp: "2026-03-23T12:00:00Z",
+		Message: &MessagePayload{
+			Role: "assistant",
+			Content: []ContentBlock{
+				{Type: "tool_use", Name: "Read"},
+			},
+			StopReason: "tool_use",
+		},
+	}}
+
+	state := DetectState(msgs, testNow)
+	if state != Working {
+		t.Errorf("expected Working, got %v", state)
+	}
+}
+
+func TestDetectState_Progress(t *testing.T) {
+	msgs := []JSONLMessage{{
+		Type:      "progress",
+		Timestamp: "2026-03-23T12:00:00Z",
+		Data:      &ProgressData{Type: "agent_progress"},
+	}}
+
+	state := DetectState(msgs, testNow)
+	if state != Working {
+		t.Errorf("expected Working, got %v", state)
+	}
+}
+
+func TestDetectState_ToolResult(t *testing.T) {
+	msgs := []JSONLMessage{{
+		Type:      "user",
+		Timestamp: "2026-03-23T12:00:00Z",
+		Message: &MessagePayload{
+			Role:    "user",
+			Content: []ContentBlock{{Type: "tool_result"}},
+		},
+	}}
+
+	state := DetectState(msgs, testNow)
+	if state != Working {
+		t.Errorf("expected Working, got %v", state)
+	}
+}
+
+func TestDetectState_IdleThreshold(t *testing.T) {
+	// Message timestamp > 60s in the past
+	msgs := []JSONLMessage{{
+		Type:      "assistant",
+		Timestamp: "2026-03-23T11:58:00Z", // 2min30s before testNow
+		Message: &MessagePayload{
+			Role:       "assistant",
+			Content:    []ContentBlock{{Type: "text", Text: "Done."}},
+			StopReason: "end_turn",
+		},
+	}}
+
+	state := DetectState(msgs, testNow)
+	if state != Idle {
+		t.Errorf("expected Idle, got %v", state)
+	}
+}
+
+func TestDetectState_EmptyMessages(t *testing.T) {
+	state := DetectState(nil, testNow)
+	if state != Unknown {
+		t.Errorf("expected Unknown, got %v", state)
+	}
+}
+
+func TestExtractPreview(t *testing.T) {
+	msgs := []JSONLMessage{
+		{
+			Type: "user",
+			Message: &MessagePayload{
+				Role:    "user",
+				Content: []ContentBlock{{Type: "text", Text: "do something"}},
+			},
+		},
+		{
+			Type: "assistant",
+			Message: &MessagePayload{
+				Role: "assistant",
+				Content: []ContentBlock{{Type: "text", Text: "Voici le resultat\nLigne 2\nLigne 3\nLigne 4\nLigne 5"}},
+			},
+		},
+	}
+
+	preview := ExtractPreview(msgs)
+	want := "Voici le resultat\nLigne 2\nLigne 3"
+	if preview != want {
+		t.Errorf("ExtractPreview = %q, want %q", preview, want)
+	}
+}
+
+func TestExtractPreview_NoAssistant(t *testing.T) {
+	msgs := []JSONLMessage{{
+		Type: "user",
+		Message: &MessagePayload{
+			Role:    "user",
+			Content: []ContentBlock{{Type: "text", Text: "hello"}},
+		},
+	}}
+
+	preview := ExtractPreview(msgs)
+	if preview != "" {
+		t.Errorf("expected empty preview, got %q", preview)
+	}
+}
+
+func TestExtractPreview_LongText(t *testing.T) {
+	// Text longer than 200 chars should be truncated
+	longText := ""
+	for i := 0; i < 100; i++ {
+		longText += "abcde "
+	}
+
+	msgs := []JSONLMessage{{
+		Type: "assistant",
+		Message: &MessagePayload{
+			Role:    "assistant",
+			Content: []ContentBlock{{Type: "text", Text: longText}},
+		},
+	}}
+
+	preview := ExtractPreview(msgs)
+	if len(preview) > 203 { // 200 + "..."
+		t.Errorf("preview too long: %d chars", len(preview))
+	}
+	if preview[len(preview)-3:] != "..." {
+		t.Errorf("expected truncation marker '...', got %q", preview[len(preview)-3:])
+	}
+}