--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-reference.rst

@@ -131,7 +131,7 @@ See :ref:`cli_query`.
       -r, --raw                   Raw output, first column of first row
       --raw-lines                 Raw output, first column of each row
       -p, --param <TEXT TEXT>...  Named :parameters for SQL query
-      --functions TEXT            Python code defining one or more custom SQL
+      --functions TEXT            Python code or file path defining custom SQL
                                   functions
       --load-extension TEXT       Path to SQLite extension, with optional
                                   :entrypoint
@@ -174,7 +174,7 @@ See :ref:`cli_memory`.
           sqlite-utils memory animals.csv --schema
 
     Options:
-      --functions TEXT            Python code defining one or more custom SQL
+      --functions TEXT            Python code or file path defining custom SQL
                                   functions
       --attach <TEXT FILE>...     Additional databases to attach - specify alias and
                                   filepath
@@ -374,7 +374,7 @@ See :ref:`cli_bulk`.
 
     Options:
       --batch-size INTEGER   Commit every X records
-      --functions TEXT       Python code defining one or more custom SQL functions
+      --functions TEXT       Python code or file path defining custom SQL functions
       --flatten              Flatten nested JSON objects, so {"a": {"b": 1}} becomes
                              {"a_b": 1}
       --nl                   Expect newline-delimited JSON