Add Linear XLSX follow-up import

AGENTS.md

@@ -344,6 +344,13 @@ batch payloads. The Revit page filters and sorts it in the client and renders a
 virtualized window. `external-object.update-planning` applies multi-object
 board, ProjectDevice and planning-value changes as one persistent command;
 linked objects are excluded because they follow their CircuitDeviceRow board.
+R9 adds the complete Linear-XLSX follow-up import. It matches Worksharing-IDs
+exactly, separates source decisions from return deviations, updates accepted
+linked quantities atomically, retains missing objects and compacts every older
+batch to metadata. `external-import.apply-follow-up` stores complete inverse
+states, so Undo/Redo never parses Excel. A separately confirmed
+`external-model-object.delete-missing` command removes missing objects and
+adjusts linked row quantities; Undo restores objects, links and quantities.
 
 The target configuration has no independent version counter; `expectedRevision`
 invalidates stale plans, and each import batch stores a configuration copy.
@@ -597,7 +604,7 @@ Critical target rules:
   selected combined/separate mode block export.
 - Missing GZF defaults visibly to 1 without separate confirmation.
 - The corrected R2 return workbook was successfully imported into Linear on
-  2026-09-30; R3 through R6 are complete and R7 is the next package.
+  2026-09-30; R3 through R9 are complete and R10 is the next package.
 - All confirmed writes use typed project commands with `expectedRevision`,
   complete inverse data and the shared project-command transaction boundary.
 - Do not add automatic circuit planning, Revit circuit adoption, multi-source