feat(ai): phase 0 couche d'outils partagee (backend/tools) + tests

- backend/tools/: ToolContext, registry @tool + schemas JSON, audit ai_tool_call
- Services lecture/recherche: list_vaults, list_directory, read_file, search_fulltext, list_tags
- Permissions check_vault_access + resolve_safe_path, confirmation gating (two-step)
- Redaction des secrets, limites de taille, audit JSONL (args sensibles resumes)
- tests/test_tools.py: 30 tests (registry, contexte, execution, confirmation, audit)
- ROADMAP: item #79 phase A livree (A2 partiel)

backend/tools/audit.py

@@ -0,0 +1,54 @@
+"""Audit logging for AI tool calls.
+
+Reuses the application audit log (``data/audit.log``, JSON lines) and adds an
+``ai_tool_call`` action. Argument values that may contain sensitive payloads
+(file content, prompts) are summarized rather than stored verbatim.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime, timezone
+from typing import Any
+
+from backend.audit import _write_entry
+
+# Argument keys whose values may contain secrets or large payloads.
+_SENSITIVE_ARG_KEYS = {"content", "text", "body"}
+_MAX_ARG_CHARS = 200
+
+
+def _sanitize_arguments(arguments: dict[str, Any] | None) -> dict[str, Any]:
+    """Return a log-safe view of tool arguments."""
+    safe: dict[str, Any] = {}
+    for key, value in (arguments or {}).items():
+        if key in _SENSITIVE_ARG_KEYS:
+            safe[key] = f"<{len(str(value))} chars>"
+        else:
+            safe[key] = str(value)[:_MAX_ARG_CHARS]
+    return safe
+
+
+def log_tool_call(
+    *,
+    username: str,
+    mode: str,
+    tool: str,
+    arguments: dict[str, Any] | None = None,
+    ok: bool = True,
+    vault: str | None = None,
+    ip: str | None = None,
+    error: str | None = None,
+) -> None:
+    """Append an ``ai_tool_call`` entry to the audit log."""
+    _write_entry({
+        "timestamp": datetime.now(timezone.utc).isoformat(),
+        "action": "ai_tool_call",
+        "username": username,
+        "ip": ip or "unknown",
+        "mode": mode,
+        "tool": tool,
+        "vault": vault,
+        "ok": ok,
+        "error": error,
+        "arguments": _sanitize_arguments(arguments),
+    })