feat(v0.3): add --project flag, CTASK_TYPE env, project root semantics

ctask new gains --project, which:
  - records type=project on the workspace
  - defaults the category to "projects"
  - applies general + project seed overlays
  - uses CTASK_PROJECT_ROOT when set, with no doubled "projects/"
    path unless the user explicitly passes -c
  - exports CTASK_TYPE=project into the child session

EnvVars now takes a taskType arg and exports CTASK_TYPE. Empty
type defaults to "task" for safety. resume/open also pass
EffectiveType so the env var is correct on resume of a v0.2
workspace.

Git init for project mode is wired in the next commit.

internal/config/config.go

@@ -60,13 +60,18 @@ func ResolveProjectRoot() string {
 }
 
 // EnvVars returns the environment variables to export into child sessions.
-func EnvVars(slug, mode, root, workspace, category string) map[string]string {
+// taskType must be "task" or "project"; an empty value defaults to "task".
+func EnvVars(slug, mode, root, workspace, category, taskType 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,
 	}
 }