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

crates/notsecrets/src/sources/prompt.rs

@@ -0,0 +1,17 @@
+use anyhow::Result;
+use crate::AgeKeySource;
+
+pub struct PromptSource;
+
+impl AgeKeySource for PromptSource {
+    fn name(&self) -> &str { "prompt" }
+
+    fn retrieve(&self) -> Result<String> {
+        let key = rpassword::prompt_password("Paste your age private key: ")
+            .map_err(|e| anyhow::anyhow!("could not read age key from prompt: {e}"))?;
+        if key.trim().is_empty() {
+            anyhow::bail!("empty age key entered");
+        }
+        Ok(key)
+    }
+}