phase 2: matrix.downgrade reads WeakModels from config

migrate the strong-model auto-downgrade gate from a hardcoded weak
list to cfg.Models.WeakModels. backward compatible: existing API
preserved, callers that don't migrate keep using DefaultWeakModels.

changes:
- internal/matrix/downgrade.go: split IsWeakModel into rule-based
  base (`:free` suffix/infix) + literal-list lookup. New
  IsWeakModelInList(model, list) takes the config-supplied list.
  DowngradeInput grows a WeakModels field; nil falls back to
  DefaultWeakModels (preserves pre-phase-2 behavior).
- internal/workflow/modes.go: add MatrixDowngradeWithWeakList(list)
  factory mirroring MatrixSearch's pattern. Plain MatrixDowngrade
  kept for backward compat.
- cmd/matrixd/main.go: handlers struct holds weakModels populated
  from cfg.Models.WeakModels at startup; handleDowngrade threads it
  into every DowngradeInput.
- cmd/observerd/main.go: registerBuiltinModes accepts weakModels
  and uses the factory variant. observerd reads cfg.Models.WeakModels
  in main().

end-to-end verified: downgrade + matrix + observer + workflow smokes
all pass. Existing TestMaybeDowngrade_TruthTable + TestIsWeakModel
unchanged (backward compat). Two new tests cover the config path:
- TestIsWeakModelInList — covers rule + literal + empty + nil
- TestMaybeDowngrade_WithConfigList — verifies cfg list overrides
  default

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/matrixd/main.go

@@ -52,7 +52,7 @@ func main() {
 	}
 
 	retriever := matrix.New(cfg.Matrixd.EmbeddURL, cfg.Matrixd.VectordURL)
-	h := &handlers{r: retriever}
+	h := &handlers{r: retriever, weakModels: cfg.Models.WeakModels}
 
 	if err := shared.Run("matrixd", cfg.Matrixd.Bind, h.register, cfg.Auth); err != nil {
 		slog.Error("server", "err", err)
@@ -62,6 +62,11 @@ func main() {
 
 type handlers struct {
 	r *matrix.Retriever
+	// weakModels comes from cfg.Models.WeakModels at startup. Threaded
+	// into every DowngradeInput so the gate uses the configured list
+	// instead of matrix.DefaultWeakModels. nil/empty falls back to
+	// the package default — matches pre-Phase 2 behavior.
+	weakModels []string
 }
 
 func (h *handlers) register(r chi.Router) {
@@ -240,6 +245,7 @@ func (h *handlers) handleDowngrade(w http.ResponseWriter, r *http.Request) {
 		// without env pollution.
 		in.ForceFullOverride = *req.ForceFullOverride
 	}
+	in.WeakModels = h.weakModels
 	writeJSON(w, http.StatusOK, matrix.MaybeDowngrade(in))
 }