feat(documents): agent tools read_workbook / read_workbook_cell / write_workbook

Exposes the typed Workbook surface to folder-mode agents. Stacks on top
of the XLSX read (osaurus-ai#929) + write (osaurus-ai#936) PRs and completes the stage-4
round-trip goal: an agent can now ingest a spreadsheet, reason about
cells and formulas in their native types, and emit a modified workbook
— all without the model having to handroll XML.

- read_workbook: returns a compact JSON summary of every sheet
  (names, row counts, merged ranges, truncated cell sample). Capped at
  200 cells per sheet so large workbooks don't blow the context
  window; agents drop to read_workbook_cell for specific values.
- read_workbook_cell: single-cell lookup by (path, sheet, A1 ref).
  Returns value, formula source, and type in a one-line JSON payload.
- write_workbook: accepts a structured sheets array and emits the
  file via XLSXEmitter. Each cell carries its A1 ref, typed value,
  and optional formula; the schema enum guards against unknown
  types. write_workbook creates parent directories and surfaces a
  sheetCount / totalCells summary on success.
- All three plug into FolderToolFactory.buildCoreTools alongside
  file_read / file_write, so they're registered the moment a working
  folder is selected and go away when it's cleared.
- Tests: 8 tests covering sheet summary rendering, missing-file and
  out-of-root rejection, formula preservation on cell lookup, missing-
  sheet error, end-to-end write + re-parse fidelity, non-xlsx path
  refusal, and empty-sheets validation. Tests reuse the sample.xlsx
  fixture from the XLSX read PR.

Author: mimeding

Packages/OsaurusCore/Folder/FolderTools.swift

@@ -1295,6 +1295,9 @@ enum FolderToolFactory {
             FileDeleteTool(rootPath: rootPath),
             DirCreateTool(rootPath: rootPath),
             BatchTool(rootPath: rootPath),
+            ReadWorkbookTool(rootPath: rootPath),
+            ReadWorkbookCellTool(rootPath: rootPath),
+            WriteWorkbookTool(rootPath: rootPath),
         ]
     }