feat: golem15.golem plugin with the OpenAI-compatible adapter and AIService.Send

- Prompt, Payload and AIResponse value objects ported from Golem15.Golem
- OpenAIAdapter: chat completions mapping with PHP's response_format,
  max_completion_tokens and reasoning_effort rules
- AIService.Send through fetchguard (TrustedMode for operator models,
  PublicOnlyMode otherwise) with PHP's failure messages
- config defaults for the SSRF allowlist and the AI timeout, lang, README

plugin.go

@@ -0,0 +1,60 @@
+// Package golem is the golem15.golem plugin: the shared AI layer of
+// SummerCMS applications. It ports the WinterCMS Golem15.Golem plugin's AI
+// service (provider adapters for the Anthropic Messages API and the
+// OpenAI-compatible chat completions API, prompts, image generation, file
+// uploads, streaming) and its SSRF guard.
+package golem
+
+import (
+	"embed"
+	"io/fs"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/pact"
+	"git.golem15.com/golem15/summercms/modules/party"
+)
+
+var (
+	_ party.Plugin   = (*Plugin)(nil)
+	_ pact.HasConfig = (*Plugin)(nil)
+	_ pact.HasLang   = (*Plugin)(nil)
+)
+
+//go:embed config
+var configFS embed.FS
+
+//go:embed lang
+var langFS embed.FS
+
+// Plugin is the golem15.golem plugin.
+type Plugin struct {
+	app *backpack.App
+}
+
+// ID is the plugin ID, also its config and lang namespace.
+func (p *Plugin) ID() string { return "golem15.golem" }
+
+// Requires is empty: the plugin depends on no other plugin.
+func (p *Plugin) Requires() []string { return nil }
+
+// Register keeps the application.
+func (p *Plugin) Register(app *backpack.App) error {
+	p.app = app
+	return nil
+}
+
+// Boot keeps the application.
+func (p *Plugin) Boot(app *backpack.App) error {
+	p.app = app
+	return nil
+}
+
+// ConfigFS ships config/config.yaml, merged under golem15.golem.*.
+func (p *Plugin) ConfigFS() fs.FS { return configFS }
+
+// LangFS ships the golem15.golem lang namespace.
+func (p *Plugin) LangFS() fs.FS { return langFS }
+
+func init() {
+	party.Register(&Plugin{})
+}