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.

cmd/open.go

@@ -38,7 +38,7 @@ func runOpen(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("updating metadata: %w", err)
 	}
 
-	envVars := config.EnvVars(ws.Meta.Slug, ws.Meta.Mode, root, ws.Path, ws.Meta.Category)
+	envVars := config.EnvVars(ws.Meta.Slug, ws.Meta.Mode, root, ws.Path, ws.Meta.Category, workspace.EffectiveType(ws.Meta))
 
 	return session.Run(session.LaunchOpts{
 		WsDir:   ws.Path,