feat(build): bake plugin metadata and the framework version into plugins.gen.go

- party.Info, party.Installed, party.Describe (additive; Plugin and Activate unchanged)
- summer build resolves plugin dirs with one go list, reads plugin.yaml, versions per D-07 (module version, git describe without --dirty, none for in-app plugins)
- build.EffectiveFramework: newest tag of a locally replaced or used framework checkout, else the go.mod require
- generated main calls party.Describe(app, PluginInfo, FrameworkVersion) before Activate; metadata failures warn and fall back to IDs
- summer new goldens and examples/hello regenerated

internal/build/testdata/new-no-plugin/plugins.gen.go.golden

@@ -2,5 +2,16 @@
 
 package main
 
+import (
+	"git.golem15.com/golem15/summercms/modules/party"
+)
+
 // PluginIDs is the ordered plugin list from summer.yaml.
 var PluginIDs = []string{}
+
+// PluginInfo is each plugin's metadata in summer.yaml order: name and
+// description from its plugin.yaml, version and source from summer build.
+var PluginInfo = []party.Info{}
+
+// FrameworkVersion is the framework version the app was built against.
+const FrameworkVersion = ""