fix: require threshold or purge_all for mimir_prune (#202)

- Add purge_all bool to PruneParams (models.rs)
- Validate in handle_prune: error if no min_decay/older_than_days/purge_all
- Prevents footgun where prune(category=X) silently archives nothing

Author: tcconnally

src/models.rs

@@ -466,6 +466,9 @@ pub struct PruneParams {
     pub limit: usize,
     #[serde(default)]
     pub dry_run: bool,
+    /// Explicitly archive everything in the category (no threshold required).
+    #[serde(default)]
+    pub purge_all: bool,
 }
 
 fn default_prune_limit() -> usize {

src/tools.rs

@@ -936,6 +936,15 @@ pub fn handle_embed(db: &Database, args: Value) -> Result<String, String> {
 pub fn handle_prune(db: &Database, args: Value) -> Result<String, String> {
     let params: PruneParams =
         serde_json::from_value(args).map_err(|e| format!("Invalid prune arguments: {}", e))?;
+
+    // #202: require a threshold or explicit purge_all — category alone is a footgun
+    if !params.purge_all && params.min_decay.is_none() && params.older_than_days.is_none() {
+        return Err(
+            "prune requires min_decay, older_than_days, or purge_all=true to archive the whole category"
+                .to_string(),
+        );
+    }
+
     match db.prune(&params) {
         Ok(report) => {
             serde_json::to_string(&report).map_err(|e| format!("Serialization failed: {}", e))