--functions can take filenames, can be used multiple times (#681)

Closes #659

The --functions option now accepts:
- File paths ending in .py (e.g., --functions my_funcs.py)
- Multiple invocations (e.g., --functions foo.py --functions 'def bar(): ...')
- Inline Python code (existing behavior)

Implementation follows the same pattern as llm's --functions flag
(simonw/llm@a880c123).

Changes:
- Added multiple=True to --functions Click option in query, bulk, and memory commands
- Modified _register_functions() to detect and read .py files
- Updated _maybe_register_functions() to iterate over multiple function sources
- Removed unused bytes/bytearray handling
- Added comprehensive tests for file paths and multiple invocations
- Updated documentation with examples

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Shorter help for --functions

---------

Co-authored-by: Claude <noreply@anthropic.com>

tests/test_cli_memory.py

@@ -307,6 +307,22 @@ def test_memory_functions():
     assert result.output.strip() == '[{"hello()": "Hello"}]'
 
 
+def test_memory_functions_multiple():
+    result = CliRunner().invoke(
+        cli.cli,
+        [
+            "memory",
+            "select triple(2), quadruple(2)",
+            "--functions",
+            "def triple(x):\n    return x * 3",
+            "--functions",
+            "def quadruple(x):\n    return x * 4",
+        ],
+    )
+    assert result.exit_code == 0
+    assert result.output.strip() == '[{"triple(2)": 6, "quadruple(2)": 8}]'
+
+
 def test_memory_return_db(tmpdir):
     # https://github.com/simonw/sqlite-utils/issues/643
     from sqlite_utils.cli import cli