refactor: reimplement digest engines as meta plugins

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

src/meta_plugin/digest.rs

@@ -0,0 +1,60 @@
+use anyhow::Result;
+use sha2::{Digest, Sha256};
+use std::io;
+use std::io::Write;
+
+use crate::meta_plugin::MetaPlugin;
+
+#[derive(Debug, Clone, Default)]
+pub struct DigestSha256MetaPlugin {
+    hasher: Sha256,
+    meta_name: String,
+}
+
+impl DigestSha256MetaPlugin {
+    pub fn new() -> DigestSha256MetaPlugin {
+        DigestSha256MetaPlugin {
+            hasher: Sha256::new(),
+            meta_name: "digest_sha256".to_string(),
+        }
+    }
+}
+
+impl MetaPlugin for DigestSha256MetaPlugin {
+    fn create(&self) -> Result<Box<dyn Write>> {
+        // For meta plugins, we don't actually create a writer since we're buffering data internally
+        // This method is required by the trait but not used in the same way as digest engines
+        Ok(Box::new(DummyWriter))
+    }
+
+    fn finalize(&mut self) -> io::Result<String> {
+        let result = self.hasher.clone().finalize();
+        Ok(format!("{:x}", result))
+    }
+
+    fn update(&mut self, data: &[u8]) {
+        self.hasher.update(data);
+    }
+
+    fn meta_name(&mut self) -> String {
+        self.meta_name.clone()
+    }
+
+    fn is_default(&self) -> bool {
+        true
+    }
+}
+
+// Dummy writer that implements Write but doesn't do anything
+// This is needed to satisfy the MetaPlugin trait requirements
+struct DummyWriter;
+
+impl Write for DummyWriter {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}