feat(v0.5): export CTASK_LAUNCH_DIR into child sessions

config.EnvVars gains a 7th launchDir argument. cmd/new, cmd/resume, and
cmd/open pass ws.Meta.LaunchDir. Child sessions can read CTASK_LAUNCH_DIR
to know which subdirectory ctask launched them into.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -90,17 +90,20 @@ func samePath(a, b string) bool {
 
 // EnvVars returns the environment variables to export into child sessions.
 // taskType must be "task" or "project"; an empty value defaults to "task".
-func EnvVars(slug, mode, root, workspace, category, taskType string) map[string]string {
+// launchDir is the workspace-relative project subdirectory for projects,
+// or empty for tasks and pre-v0.5 projects.
+func EnvVars(slug, mode, root, workspace, category, taskType, launchDir string) map[string]string {
 	if taskType == "" {
 		taskType = "task"
 	}
 	return map[string]string{
-		"CTASK_TASK":      slug,
-		"CTASK_MODE":      mode,
-		"CTASK_ROOT":      root,
-		"CTASK_WORKSPACE": workspace,
-		"CTASK_CATEGORY":  category,
-		"CTASK_TYPE":      taskType,
+		"CTASK_TASK":       slug,
+		"CTASK_MODE":       mode,
+		"CTASK_ROOT":       root,
+		"CTASK_WORKSPACE":  workspace,
+		"CTASK_CATEGORY":   category,
+		"CTASK_TYPE":       taskType,
+		"CTASK_LAUNCH_DIR": launchDir,
 	}
 }