feat: project init with config package and root command

Go module, cobra root command, config resolution (CTASK_ROOT, CTASK_AGENT, EnvVars) with tests.

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

internal/config/config.go

@@ -0,0 +1,48 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// ResolveRoot returns the absolute workspace root path.
+// Reads CTASK_ROOT env var, falls back to ~/ai-workspaces (or %USERPROFILE%\ai-workspaces on Windows).
+func ResolveRoot() string {
+	root := os.Getenv("CTASK_ROOT")
+	if root == "" {
+		home, _ := os.UserHomeDir()
+		return filepath.Join(home, "ai-workspaces")
+	}
+	// Expand leading tilde
+	if strings.HasPrefix(root, "~/") || root == "~" {
+		home, _ := os.UserHomeDir()
+		root = filepath.Join(home, root[2:])
+	}
+	abs, err := filepath.Abs(root)
+	if err != nil {
+		return root
+	}
+	return abs
+}
+
+// ResolveAgent returns the agent command.
+// Reads CTASK_AGENT env var, falls back to "claude".
+func ResolveAgent() string {
+	agent := os.Getenv("CTASK_AGENT")
+	if agent == "" {
+		return "claude"
+	}
+	return agent
+}
+
+// EnvVars returns the environment variables to export into child sessions.
+func EnvVars(slug, mode, root, workspace, category string) map[string]string {
+	return map[string]string{
+		"CTASK_TASK":      slug,
+		"CTASK_MODE":      mode,
+		"CTASK_ROOT":      root,
+		"CTASK_WORKSPACE": workspace,
+		"CTASK_CATEGORY":  category,
+	}
+}