docs(custom_tool): document --tools file loading and /tools load command (gptme#1547)

* docs(custom_tool): document --tools file loading and /tools load command

PR gptme#1540 added support for loading custom tools from .py files via
--tools flag, but the docs still only covered TOOL_MODULES registration.
Added documentation for both the --tools CLI flag and /tools load runtime
command, reorganized the "Registering the Tool" section into clearer
subsections for file-based vs module-based loading.

* docs(custom_tool): fix --tools examples and remove incorrect /tools load usage

- Add '+' prefix examples (additive mode) as the recommended approach
- Explain that without '+', --tools replaces the entire default toolset
- Remove incorrect '/tools load ./my_tool.py' example — /tools load only
  accepts tool names (e.g. 'browser'), not file paths

Fixes feedback from Greptile and Erik in PR review.

* docs(custom_tool): fix multi-file example — one '+' is enough for additive mode

---------

Co-authored-by: TimeToBuildBob <TimeToBuildBob@users.noreply.github.com>

Author: TimeToBuildBob

docs/custom_tool.rst

@@ -213,10 +213,38 @@ Use **custom tools** when you need:
 - Complex argument validation
 - Operation without shell access
 
-Registering the Tool
+Use ``--tools +./file.py`` for **quick development** — no packaging or ``PYTHONPATH`` needed. Use ``TOOL_MODULES`` for **production** tools that are installed as packages.
+
+Loading Custom Tools
 --------------------
-To ensure your tool is available for use, you can specify the module in the ``TOOL_MODULES`` env variable or
-setting in your :doc:`project configuration file <config>`, which will automatically load your custom tools.
+
+There are two ways to load custom tools: directly from a ``.py`` file (quickest for development), or via a Python module (best for packaged/shared tools).
+
+Loading from a File (``--tools``)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The simplest way to use a custom tool is to pass the file path directly via ``--tools``:
+
+.. code-block:: bash
+
+    # Add your tool on top of the default toolset (most common)
+    gptme --tools +./my_tool.py "use the my_tool tool"
+
+    # Replace the default toolset entirely with specific tools
+    gptme --tools save,patch,./my_tool.py "prompt"
+
+    # Load multiple custom tool files (additive — one '+' is enough)
+    gptme --tools +./tools/search.py,./tools/deploy.py "prompt"
+
+The ``+`` prefix adds tools on top of the default toolset. Without ``+``, the argument replaces the entire toolset — so you'd need to explicitly list any built-in tools you still want.
+
+**How it works**: gptme scans the file for top-level ``ToolSpec`` instances and registers them. The file must be a regular ``.py`` file containing at least one ``ToolSpec`` variable (see the example in `Creating a Custom Tool`_ above).
+
+Loading from a Module (``TOOL_MODULES``)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For packaged tools, specify the module in the ``TOOL_MODULES`` env variable or
+in your :doc:`project configuration file <config>`:
 
 .. code-block:: toml