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]>

backend/tools/labels.py

@@ -52,6 +52,10 @@ _STEP_LABELS: dict[str, tuple[str, str | None]] = {
     "git_search_issues": ("git_issues", "query"),
     "git_get_file": ("git_file", "path"),
     "create_xlsx": ("xlsx_create", "path"),
+    "list_xlsx_sheets": ("xlsx_sheets", "path"),
+    "xlsx_to_markdown": ("xlsx_read", "path"),
+    "update_xlsx_cells": ("xlsx_update", "path"),
+    "append_xlsx_rows": ("xlsx_append", "path"),
     "create_docx": ("docx_create", "path"),
     "create_csv": ("csv_create", "path"),
     "create_pdf": ("pdf_create", "path"),