feat: outils IA de lecture et d'edition des classeurs existants #153

A6 : list_xlsx_sheets (noms + dimensions + truncated), xlsx_to_markdown
(table bornee 100x20 pour le contexte LLM), update_xlsx_cells et
append_xlsx_rows (WRITE + confirmation, via edit_xlsx_cells : verrou,
ecriture atomique, neutralisation des formules, 409 lossy). Les lignes
ajoutees sont coercees comme dans le viewer (A10). Labels de step
ai.step.xlsx_* FR/EN ; update_xlsx_cells/append_xlsx_rows branches sur
MUTATING_TOOLS et FILE_WRITE_TOOLS (refresh viewer via obsigate:file-written).

Tests : test_spreadsheet_tools.py 17 (risque, confirmation, persistance,
garde formule heritee, coercion) + contre-preuve (cells vide -> 2 echecs).
ruff 0, mypy 0, i18n parity, validate-imports.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <[email protected]>

frontend/js/bookslm.js

@@ -60,12 +60,14 @@ const MUTATING_TOOLS = new Set([
   'rename_file', 'rename_directory', 'move_path', 'replace_in_files',
   'delete_file', 'delete_directory', 'restore_backup',
   'create_xlsx', 'create_docx', 'create_csv', 'create_pdf',
+  'update_xlsx_cells', 'append_xlsx_rows',
 ]);
 // Subset carrying a concrete `vault` + `path`: the displayed document is
 // reloaded from disk so an open viewer/editor reflects the agent's write.
 const FILE_WRITE_TOOLS = new Set([
   'edit_file', 'append_to_file', 'create_file', 'restore_backup',
   'create_xlsx', 'create_docx', 'create_csv', 'create_pdf',
+  'update_xlsx_cells', 'append_xlsx_rows',
 ]);
 
 /**