--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>

docs/cli.rst

@@ -368,6 +368,22 @@ This example defines a function which extracts the domain from a URL:
 
 Every callable object defined in the block will be registered as a SQL function with the same name, with the exception of functions with names that begin with an underscore.
 
+You can also pass the path to a Python file containing function definitions:
+
+.. code-block:: bash
+
+    sqlite-utils query sites.db "select url, domain(url) from urls" --functions functions.py
+
+The ``--functions`` option can be used multiple times to load functions from multiple sources:
+
+.. code-block:: bash
+
+    sqlite-utils query sites.db "select url, domain(url), extract_path(url) from urls" \
+      --functions domain_funcs.py \
+      --functions 'def extract_path(url):
+        from urllib.parse import urlparse
+        return urlparse(url).path'
+
 .. _cli_query_extensions:
 
 SQLite extensions