feat(notsecrets): add age key retrieval with bw/file/prompt sources

crates/notsecrets/src/sources/file.rs

@@ -0,0 +1,22 @@
+use std::path::PathBuf;
+use anyhow::Result;
+use crate::AgeKeySource;
+
+pub struct FileSource {
+    path: PathBuf,
+}
+
+impl FileSource {
+    pub fn new(path: PathBuf) -> Self {
+        Self { path }
+    }
+}
+
+impl AgeKeySource for FileSource {
+    fn name(&self) -> &str { "file" }
+
+    fn retrieve(&self) -> Result<String> {
+        std::fs::read_to_string(&self.path)
+            .map_err(|e| anyhow::anyhow!("cannot read key file {}: {e}", self.path.display()))
+    }
+}