refactor(tools): add src/tools/ package with shared _common

Slice 1 Task 2 (#4082/#4071). Adds the package skeleton and moves the
shared _parse_tool_args helper into src/tools/_common.py. Domain modules
will import from here. tool_implementations.py is untouched at this step.

src/tools/__init__.py

@@ -0,0 +1,6 @@
+"""Tool implementation package, split by domain (slice 1, #4082/#4071).
+
+Public tool functions live in domain modules. ``src.tool_implementations``
+re-exports from here for backward compatibility.
+"""
+from src.tools._common import _parse_tool_args  # noqa: F401

src/tools/_common.py

@@ -0,0 +1,39 @@
+"""Shared helpers used across tool implementation domains.
+
+Extracted from tool_implementations.py as part of slice 1 (#4082/#4071).
+Domain modules under src/tools/ import from here.
+"""
+import json
+
+
+def _parse_tool_args(content):
+    """Parse a tool-call argument blob.
+
+    Accepts either a JSON string or an already-decoded dict. Unwraps the
+    common `{"body": {...}}` envelope that smaller models emit when they
+    read tool descriptions like "Body is JSON: {...}" literally — they
+    pass `body` as a field name rather than treating it as a noun.
+
+    Returns a dict on success, raises ValueError on bad JSON.
+    """
+    if isinstance(content, str):
+        try:
+            args = json.loads(content) if content.strip() else {}
+        except (json.JSONDecodeError, TypeError) as e:
+            raise ValueError(str(e))
+    elif isinstance(content, dict):
+        args = content
+    else:
+        args = {}
+    # Unwrap {"body": {...}} envelope — but only if `body` is the sole key
+    # and points at a dict. We don't want to clobber a legitimate `body`
+    # field on tools where it's a real arg (e.g. send_email body text).
+    if (
+        isinstance(args, dict)
+        and len(args) == 1
+        and "body" in args
+        and isinstance(args["body"], dict)
+        and "action" in args["body"]  # extra safety: only unwrap if the inner dict looks like a tool call
+    ):
+        args = args["body"]
+    return args