--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_bulk.py

@@ -45,6 +45,32 @@ def test_cli_bulk(test_db_and_path):
     ] == list(db["example"].rows)
 
 
+def test_cli_bulk_multiple_functions(test_db_and_path):
+    db, db_path = test_db_and_path
+    result = CliRunner().invoke(
+        cli.cli,
+        [
+            "bulk",
+            db_path,
+            "insert into example (id, name) values (:id, myupper(mylower(:name)))",
+            "-",
+            "--nl",
+            "--functions",
+            "myupper = lambda s: s.upper()",
+            "--functions",
+            "mylower = lambda s: s.lower()",
+        ],
+        input='{"id": 3, "name": "ThReE"}\n{"id": 4, "name": "FoUr"}\n',
+    )
+    assert result.exit_code == 0, result.output
+    assert [
+        {"id": 1, "name": "One"},
+        {"id": 2, "name": "Two"},
+        {"id": 3, "name": "THREE"},
+        {"id": 4, "name": "FOUR"},
+    ] == list(db["example"].rows)
+
+
 def test_cli_bulk_batch_size(test_db_and_path):
     db, db_path = test_db_and_path
     proc = subprocess.Popen(