v2: Observing log (Org files) !23

• ObservingLog package: one Org file per observation (* Object, [YYYY-MM-DD Day HH:MM], property drawer, notes), parse and write, replace by :ID:.
• App: log list/editor; "Log" on Sky cards pre-fills the entry; files in Documents/Observations, shown in Files.

Tests: round-trip and folder save/list/delete. Checked in the simulator: Uranus entry saved, edited in place, file contents as expected.