feat: suffix-based chat agent selection (#23741)

Adds suffix-based agent selection for chatd. Template authors can direct
chat traffic to a specific root workspace agent by naming it with the
`-coderd-chat` suffix (for example, `coder_agent "dev-coderd-chat"`).
When no suffix match exists, chatd falls back to the first root agent by
`DisplayOrder`, then `Name`. Multiple suffix matches return an error.

The selection logic lives in `coderd/x/chatd/internal/agentselect` and
is shared by chatd core plus the workspace chat tools so all chat entry
points pick the same agent deterministically.

No database migrations, API contract changes, or provider changes. The
experimental sandbox template was split out to #23777.

coderd/x/chatd/chattool/startworkspace.go

@@ -9,6 +9,7 @@ import (
 	"golang.org/x/xerrors"
 
 	"github.com/coder/coder/v2/coderd/database"
+	"github.com/coder/coder/v2/coderd/x/chatd/internal/agentselect"
 	"github.com/coder/coder/v2/codersdk"
 )
 
@@ -144,7 +145,7 @@ func StartWorkspace(options StartWorkspaceOptions) fantasy.AgentTool {
 	)
 }
 
-// waitForAgentAndRespond looks up the first agent in the workspace's
+// waitForAgentAndRespond selects the chat agent from the workspace's
 // latest build, waits for it to become reachable, and returns a
 // success response.
 func waitForAgentAndRespond(
@@ -155,7 +156,7 @@ func waitForAgentAndRespond(
 ) (fantasy.ToolResponse, error) {
 	agents, err := db.GetWorkspaceAgentsInLatestBuildByWorkspaceID(ctx, ws.ID)
 	if err != nil || len(agents) == 0 {
-		// Workspace started but no agent found — still report
+		// Workspace started but no agent found - still report
 		// success so the model knows the workspace is up.
 		return toolResponse(map[string]any{
 			"started":        true,
@@ -164,11 +165,21 @@ func waitForAgentAndRespond(
 		}), nil
 	}
 
+	selected, err := agentselect.FindChatAgent(agents)
+	if err != nil {
+		return toolResponse(map[string]any{
+			"started":        true,
+			"workspace_name": ws.Name,
+			"agent_status":   "selection_error",
+			"agent_error":    err.Error(),
+		}), nil
+	}
+
 	result := map[string]any{
 		"started":        true,
 		"workspace_name": ws.Name,
 	}
-	for k, v := range waitForAgentReady(ctx, db, agents[0].ID, agentConnFn) {
+	for k, v := range waitForAgentReady(ctx, db, selected.ID, agentConnFn) {
 		result[k] = v
 	}
 	return toolResponse(result), nil