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>

internal/matrix/downgrade.go

@@ -37,6 +37,12 @@ const (
 // diagnostic runs ("LH_FORCE_FULL_ENRICHMENT=1" or "true").
 const EnvForceFullEnrichment = "LH_FORCE_FULL_ENRICHMENT"
 
+// DefaultWeakModels is the fallback weak-model list used when no
+// config-driven list is provided. Matches the pre-config hardcoded
+// switch — preserved for backward compatibility with callers that
+// haven't migrated to the [models] tier config (Phase 1).
+var DefaultWeakModels = []string{"qwen3.5:latest", "qwen3:latest"}
+
 // IsWeakModel returns true for models matrix-corpus composition
 // demonstrably helped during the 2026-04-26 pass5 bake-off. Strong
 // models (default) get matrix dropped to avoid the "composed lost
@@ -45,17 +51,29 @@ const EnvForceFullEnrichment = "LH_FORCE_FULL_ENRICHMENT"
 // Weak signals:
 //   - `:free` suffix (OpenRouter free tier, e.g. `gpt-oss-120b:free`)
 //   - `:free/` infix (handles routing-prefixed names like `or:free/x`)
-//   - `qwen3.5:latest` / `qwen3:latest` — local last-resort rung
+//   - DefaultWeakModels list — local-hot-path eligible.
 //
-// Add new weak models by extending this function alongside variance
-// data that justifies it.
+// For config-driven lists, use IsWeakModelInList.
 func IsWeakModel(model string) bool {
+	return IsWeakModelInList(model, DefaultWeakModels)
+}
+
+// IsWeakModelInList is the config-aware variant. Callers pass the
+// configured weak-model list (typically cfg.Models.WeakModels) and
+// the function applies the same rule-based checks (`:free` suffix /
+// infix) plus the literal list match.
+//
+// Empty list still applies the rule-based checks — only the literal
+// match becomes a no-op. That preserves "free-tier always weak"
+// regardless of what the operator configured.
+func IsWeakModelInList(model string, weakList []string) bool {
 	if strings.HasSuffix(model, ":free") || strings.Contains(model, ":free/") {
 		return true
 	}
-	switch model {
-	case "qwen3.5:latest", "qwen3:latest":
-		return true
+	for _, w := range weakList {
+		if w == model {
+			return true
+		}
 	}
 	return false
 }
@@ -75,6 +93,10 @@ type DowngradeInput struct {
 	Model             string
 	ForcedMode        bool
 	ForceFullOverride bool
+	// WeakModels overrides the default weak-model list (DefaultWeakModels)
+	// when non-nil. Typically populated from cfg.Models.WeakModels at
+	// startup. nil = use DefaultWeakModels (backward compat).
+	WeakModels []string
 }
 
 // DowngradeDecision is the output. DowngradedFrom is non-empty
@@ -106,7 +128,11 @@ func MaybeDowngrade(in DowngradeInput) DowngradeDecision {
 		out.Reason = EnvForceFullEnrichment + " bypass"
 		return out
 	}
-	if IsWeakModel(in.Model) {
+	weakList := in.WeakModels
+	if weakList == nil {
+		weakList = DefaultWeakModels
+	}
+	if IsWeakModelInList(in.Model, weakList) {
 		out.Reason = "weak model; matrix composition demonstrably helped (2026-04-26 free-tier bake-off)"
 		return out
 	}