implement abstract node/directory tree
refactor dow.os.main to use more :as than :refer
move get-apex from dow.ext.context to dow.tree
implement dow.tree/get-nodes-by-path and tests
fix tree/get-root by expanding home tilde on :root
move context/prj-specific helpers out of dow.tree
generate list of contexts at ~/.dow/contexts
refactor dow-setting to act as getter and setter
fix get-apex to return full path
fix dowfile format to use org's '*' as headliner
implement dow.capture proof-of-concept