fix: unconscious agent cycling

- Read max_concurrent from config (llm_concurrency) instead of hardcoding 2
- Add not-visited: and visited: filters to query parser (were in engine
  but missing from parser after unification)

The organize agent was stuck in a spawn/fail loop because its query used
not-visited: which the parser didn't recognize.

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

src/hippocampus/query/parser.rs

@@ -99,6 +99,8 @@ peg::parser! {
             / "age:" c:cmp_duration() { Stage::Filter(Filter::Age(c)) }
             / "key:" g:ident() { Stage::Filter(Filter::KeyGlob(g)) }
             / "provenance:" p:ident() { Stage::Filter(Filter::Provenance(p)) }
+            / "not-visited:" a:ident() "," d:integer() { Stage::Filter(Filter::NotVisited { agent: a, duration: d as i64 }) }
+            / "visited:" a:ident() { Stage::Filter(Filter::Visited { agent: a }) }
             / "all" { Stage::Generator(Generator::All) }
             // Graph algorithms
             / "spread" { Stage::Algorithm(AlgoStage { algo: Algorithm::Spread, params: std::collections::HashMap::new() }) }