feat: add len variable for tier conditions and LLM prompt helper

pkg/billingexpr/run.go

@@ -13,7 +13,8 @@ import (
 
 // RunExpr compiles (with cache) and executes an expression string.
 // The environment exposes:
-//   - p, c             — prompt / completion tokens
+//   - p, c             — prompt / completion tokens (auto-excluding separately-priced sub-categories)
+//   - len              — total input context length for tier conditions (never reduced by sub-category exclusion)
 //   - cr, cc, cc1h     — cache read / creation / creation-1h tokens
 //   - tier(name, value) — trace callback that records which tier matched
 //   - max, min, abs, ceil, floor — standard math helpers
@@ -54,6 +55,7 @@ func runProgram(prog *vm.Program, params TokenParams, request RequestInput) (flo
 	env := map[string]interface{}{
 		"p":    params.P,
 		"c":    params.C,
+		"len":  params.Len,
 		"cr":   params.CR,
 		"cc":   params.CC,
 		"cc1h": params.CC1h,