P1: model layer (convar->config inversion) + llmmeta

Lifts mort's pkg/logic/llms into executus/model, decoupled from mort:

- tiers.go: the tier resolver now reads a host-supplied config.Source under
  "model.tier.<name>" with host-supplied fallbacks (Configure(cfg, defaults,
  ttl)), instead of convar.Manager. Tier NAMES + specs are host config; the
  resolution mechanism (cache, reasoning-suffix dialect, chain validation) is
  generic. No tier names hard-coded in the harness.
- sink.go: usage/trace recording inverted off mort's llmusage/llmtrace into
  UsageSink / TraceSink seams + a model-owned Span, with nil-safe context
  attribution helpers (WithModel/WithTraceID/WithUsageTool/WithUsageUser).
  Both sinks optional (nil = off) so a light host records nothing.
- lane decoration repointed to executus/lane; utils.Errorf -> fmt.Errorf.
- call.go keeps GenerateWith[T] (instrumented structured output) — this is the
  structured-output primitive; no separate structured/ package.
- llmmeta moved over model/ (the meta-LLM helper: tier allowlist + JSON retry
  + ledger). Its tests configure a minimal tier table via TestMain.

New tests cover the inversion: config overrides fallback, tier registration,
reasoning-suffix survival, nested-tier rejection, nil-sink no-ops.

Full module: go build/vet/test -race green; core go.sum still free of
gorm/redis/discordgo/sqlite.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

llmmeta/main_test.go

@@ -0,0 +1,21 @@
+package llmmeta
+
+import (
+	"os"
+	"testing"
+	"time"
+
+	"gitea.stevedudenhoeffer.com/steve/executus/model"
+)
+
+// TestMain configures a minimal model tier table so the helper's
+// model.ParseModelForContext("fast"/"standard") resolves. The actual LLM call
+// is stubbed per-test via SetCompleteForTest, so these specs are only parsed
+// (anthropic registers with an empty key and errors at call time, not parse).
+func TestMain(m *testing.M) {
+	model.Configure(nil, map[string]string{
+		"fast":     "anthropic/claude-haiku-4-5",
+		"standard": "anthropic/claude-sonnet-4-6",
+	}, time.Minute)
+	os.Exit(m.Run())
+}