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.go

@@ -0,0 +1,83 @@
+package main
+
+import (
+	"strings"
+	"time"
+)
+
+const IdleThreshold = 60 * time.Second
+
+// DetectState determines the session state from the last JSONL messages.
+// The now parameter enables deterministic testing of the idle threshold.
+func DetectState(messages []JSONLMessage, now time.Time) SessionState {
+	if len(messages) == 0 {
+		return Unknown
+	}
+
+	last := messages[len(messages)-1]
+
+	// Check idle threshold first
+	if ts, err := time.Parse(time.RFC3339, last.Timestamp); err == nil {
+		if now.Sub(ts) > IdleThreshold {
+			return Idle
+		}
+	}
+
+	if last.Type == "assistant" && last.Message != nil {
+		switch last.Message.StopReason {
+		case "end_turn":
+			return NeedsInput
+		case "tool_use":
+			for _, block := range last.Message.Content {
+				if block.Type == "tool_use" && block.Name == "AskUserQuestion" {
+					return NeedsInput
+				}
+			}
+			return Working
+		}
+	}
+
+	if last.Type == "progress" {
+		return Working
+	}
+
+	if last.Type == "user" && last.Message != nil {
+		for _, block := range last.Message.Content {
+			if block.Type == "tool_result" {
+				return Working
+			}
+		}
+	}
+
+	return Unknown
+}
+
+// ExtractPreview finds the last assistant text content and returns the first
+// 3 lines, truncated to 200 characters.
+func ExtractPreview(messages []JSONLMessage) string {
+	for i := len(messages) - 1; i >= 0; i-- {
+		msg := messages[i]
+		if msg.Type != "assistant" || msg.Message == nil {
+			continue
+		}
+		for _, block := range msg.Message.Content {
+			if block.Type == "text" && block.Text != "" {
+				return truncatePreview(block.Text)
+			}
+		}
+	}
+	return ""
+}
+
+func truncatePreview(text string) string {
+	lines := strings.SplitN(text, "\n", 4)
+	if len(lines) > 3 {
+		lines = lines[:3]
+	}
+	result := strings.Join(lines, "\n")
+
+	if len(result) > 200 {
+		result = result[:200] + "..."
+	}
+	return result
+}