refactor(agent): update agentcontainers api initialization (#17600)

There were too many ways to configure the agentcontainers API resulting
in inconsistent behavior or features not being enabled. This refactor
introduces a control flag for enabling or disabling the containers API.
When disabled, all implementations are no-op and explicit endpoint
behaviors are defined. When enabled, concrete implementations are used
by default but can be overridden by passing options.

cli/ssh.go

@@ -299,8 +299,6 @@ func (r *RootCmd) ssh() *serpent.Command {
 				}
 				if len(cts.Containers) == 0 {
 					cliui.Info(inv.Stderr, "No containers found!")
-					cliui.Info(inv.Stderr, "Tip: Agent container integration is experimental and not enabled by default.")
-					cliui.Info(inv.Stderr, "     To enable it, set CODER_AGENT_DEVCONTAINERS_ENABLE=true in your template.")
 					return nil
 				}
 				var found bool