docs: improve documentation for ProgramWriter and plugins module

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/plugins.rs

@@ -1,6 +1,16 @@
-pub(crate) use std::io::Write;
+//! Shared plugin utilities for the keep application.
+//!
+//! This module provides common functionality that can be used by different
+//! plugin implementations throughout the application.
 
+use std::io::Write;
+
+/// A wrapper around a child process's stdin that implements the Write trait.
+///
+/// This struct allows writing data to an external process's standard input
+/// in a way that's compatible with Rust's I/O traits.
 pub struct ProgramWriter {
+    /// The stdin handle of a spawned child process
     pub stdin: std::process::ChildStdin,
 }