fix: drop groups emptied in the interactive editor instead of aborting the session

[vitali87]

Summary

Moving every hunk out of a group in the interactive editor ended the session with Groups ['pr-2'] are empty after editing. and exit 1 — all other edits were thrown away. Dissolving a group is a legitimate edit.

split now drops emptied groups (Dropped empty group(s) after editing: pr-2), rewires their dependants to the nearest kept ancestors (chains of emptied groups collapse transitively; a dropped root leaves its dependants as roots; diamonds keep both surviving parents), then re-validates the reduced plan. It only errors when every group is empty. README's editor paragraph now describes this.

Stacked on #60 (stack #74 = #58→#60→#129) because #60 recomputes LOC after each move — without it the dropped group's stale estimated_loc made validate_loc fail.

Test plan

• TestDropEmptyGroups: no-op, single drop with inheritance, dropped root, transitive chain (unmasked), direct-edge dedup, diamond through dropped groups
• TestEditorEmptiedGroupEndToEnd: unmocked _interactive_edit → drop → real validate_plan returns []
• TestSplitEditorEmptiedGroup: CLI saves the one-group plan; all-empty exits 1
• Review verified the e2e fixture (move a.py:2 pr-2 pr-1, done) now exits 0 with a valid one-group plan
• 470 tests pass, ruff clean
• Local review: 5/5 (after three fix→re-review rounds)

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