feat: opencode zen adapter + first-run config generation, fix stats/stream bugs

- adapters/opencode.lua: opencode.ai zen free pool adapter — sends the
  opencode client User-Agent (zen fingerprints clients by UA; non-official
  clients hit FreeUsageLimitError); pairs with api_key: public
- config: no config file ships in the repo; first run generates a default
  config at the -config path with a random admin key, loopback listen and a
  keyless zen source (config.EnsureDefault); remove config.example.yaml
- lua: seed bundled adapters from the embedded FS instead of a hardcoded
  name list
- ui: widen model kind select (chat was clipped to 'cha')
- phase 5 bugfixes: stats ms/s bucket mixing, cleanScopes nil, ctx.Err
  guards, direct-path ModelAvailable, empty stream body failure,
  bestImageModel rewrite, transform failure recording, Core.mu, timer,
  effective model for tool-calls

internal/gateway/chat.go

@@ -100,7 +100,10 @@ func (g *Gateway) resolveCands(ctx context.Context, req *chatRequest) ([]*provid
 		return nil, effective
 	}
 	first := cands[0]
-	eff := first.ModelFor(model)
+	// use the prefix-stripped id (effective), never the raw "src:model" form:
+	// ModelFor does exact matching and would fall back to the source's best
+	// chat model for an unknown id
+	eff := first.ModelFor(effective)
 	if eff == "" {
 		eff = firstModel(first)
 	}