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/observerd/main.go

@@ -74,7 +74,7 @@ func main() {
 	// to gateway's matrixd_url so a single-toml deploy works without
 	// duplicating the address.
 	matrixdURL := cfg.Gateway.MatrixdURL
-	registerBuiltinModes(runner, matrixdURL)
+	registerBuiltinModes(runner, matrixdURL, cfg.Models.WeakModels)
 
 	h := &handlers{store: store, runner: runner}
 	if err := shared.Run("observerd", cfg.Observerd.Bind, h.register, cfg.Auth); err != nil {
@@ -204,7 +204,7 @@ func summarizeOutput(output map[string]any) string {
 //   - playbook.record (HTTP to matrixd)
 //   - playbook.lookup (HTTP to matrixd)
 //   - llm.chat (HTTP to gateway /v1/chat)
-func registerBuiltinModes(r *workflow.Runner, matrixdURL string) {
+func registerBuiltinModes(r *workflow.Runner, matrixdURL string, weakModels []string) {
 	// Fixture modes for runner mechanics smokes.
 	r.RegisterMode("fixture.echo", func(_ workflow.Context, input map[string]any) (map[string]any, error) {
 		out := make(map[string]any, len(input))
@@ -220,7 +220,10 @@ func registerBuiltinModes(r *workflow.Runner, matrixdURL string) {
 
 	// Real modes — pure-function wrappers (no I/O).
 	r.RegisterMode("matrix.relevance", workflow.MatrixRelevance)
-	r.RegisterMode("matrix.downgrade", workflow.MatrixDowngrade)
+	// matrix.downgrade reads weakModels from config — Phase 2.
+	// nil/empty falls back to matrix.DefaultWeakModels per the
+	// MatrixDowngradeWithWeakList factory contract.
+	r.RegisterMode("matrix.downgrade", workflow.MatrixDowngradeWithWeakList(weakModels))
 	r.RegisterMode("distillation.score", workflow.DistillationScore)
 	r.RegisterMode("drift.scorer", workflow.DriftScorer)