feat: add experimental agents support (#22290)

feat: add AI chat system with agent tools and chat UI

Introduce the chatd subsystem and Agents UI for AI-powered chat
within Coder workspaces.

- Add chatd package with chat loop, message compaction, prompt
  management, and LLM provider integration (OpenAI, Anthropic)
- Add agent tools: create workspace, list/read templates, read/write/
  edit files, execute commands
- Add chat API endpoints with streaming, message editing, and
  durable reconnection
- Add database schema and migrations for chats, chat messages, chat
  providers, and chat model configs
- Add RBAC policies and dbauthz enforcement for chat resources
- Add Agents UI pages with conversation timeline, queued messages
  list, diff viewer, and model configuration panel
- Add comprehensive test coverage including coderd integration tests,
  chatd unit tests, and Storybook stories
- Gate feature behind experiments flag

---------

Co-authored-by: Cian Johnston <cian@coder.com>
Co-authored-by: Danielle Maywood <danielle@themaywoods.com>
Co-authored-by: Jeremy Ruppel <jeremy@coder.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

coderd/chatd/chattool/chattool.go

@@ -0,0 +1,33 @@
+package chattool
+
+import (
+	"encoding/json"
+	"unicode/utf8"
+
+	"charm.land/fantasy"
+)
+
+// toolResponse builds a fantasy.ToolResponse from a JSON-serializable
+// result payload.
+func toolResponse(result map[string]any) fantasy.ToolResponse {
+	data, err := json.Marshal(result)
+	if err != nil {
+		return fantasy.NewTextResponse("{}")
+	}
+	return fantasy.NewTextResponse(string(data))
+}
+
+func truncateRunes(value string, maxLen int) string {
+	if maxLen <= 0 || value == "" {
+		return ""
+	}
+	if utf8.RuneCountInString(value) <= maxLen {
+		return value
+	}
+
+	runes := []rune(value)
+	if maxLen > len(runes) {
+		maxLen = len(runes)
+	}
+	return string(runes[:maxLen])
+}