docs: add prompts, tools, and settings documentation

Add custom prompts documentation, tools configuration guide, and
settings reference. Add design document for custom prompts and tools
RFC. Update README with tools and prompts configuration sections.

Assisted-by: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: derekbarbosa <derekasobrab@gmail.com>

Author: derekbarbosa

README.md

@@ -91,6 +91,10 @@ cd sashiko
 Copy an example config to get started. For a full reference of every
 setting, see the [Configuration Reference](docs/configuration.md).
 
+Optional customization sections:
+*   **Tools**: Configure which AI tools are enabled (optional). See [docs/tools.md](docs/tools.md) for details.
+*   **Prompts**: Customize review stages and prompts (optional). See [docs/prompts.md](docs/prompts.md) for details.
+
 #### Configuring the LLM Provider
 
 Sashiko supports multiple LLM providers. To get started with the default