query: unify PEG and engine parsers

PEG parser now handles both expression syntax (degree > 5 | sort degree)
and pipeline syntax (all | type:episodic | sort:timestamp). Deleted
Stage::parse() and helpers from engine.rs — it's now pure execution.

All callers use parse_stages() from parser.rs as the single entry point.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/subconscious/defs.rs

@@ -801,7 +801,7 @@ pub fn run_agent(
 
     // Run the query if present
     let keys = if !def.query.is_empty() {
-        let mut stages = search::Stage::parse_pipeline(&def.query)?;
+        let mut stages = crate::query_parser::parse_stages(&def.query)?;
         let has_limit = stages.iter().any(|s|
             matches!(s, search::Stage::Transform(search::Transform::Limit(_))));
         if !has_limit {