feat(cli): add dynamic completions for ssh command (#20171)

Adds CompletionHandler to the ssh command that dynamically suggests
workspace and agent targets based on the user's running workspaces.

Features:
- Suggests workspace name for single-agent workspaces
- Suggests agent.workspace format for all agents in multi-agent
workspaces
- Only shows running workspaces (matches immediate availability)
- Alphabetically sorted completions for better UX

Tests cover single-agent, multi-agent, and network error scenarios.

Amp-Thread-ID:
https://ampcode.com/threads/T-d137d343-53f3-4ece-be5a-584249bbd9e8

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closes #20158

Demo:


https://github.com/user-attachments/assets/e1000463-ded6-4bc9-b013-61780453f019

---------

Co-authored-by: Ethan Dickson <ethan@coder.com>

cli/ssh.go

@@ -109,6 +109,51 @@ func (r *RootCmd) ssh() *serpent.Command {
 				}
 			},
 		),
+		CompletionHandler: func(inv *serpent.Invocation) []string {
+			client, err := r.InitClient(inv)
+			if err != nil {
+				return []string{}
+			}
+
+			res, err := client.Workspaces(inv.Context(), codersdk.WorkspaceFilter{
+				Owner: codersdk.Me,
+			})
+			if err != nil {
+				return []string{}
+			}
+
+			var mu sync.Mutex
+			var completions []string
+			var wg sync.WaitGroup
+			for _, ws := range res.Workspaces {
+				wg.Add(1)
+				go func() {
+					defer wg.Done()
+					resources, err := client.TemplateVersionResources(inv.Context(), ws.LatestBuild.TemplateVersionID)
+					if err != nil {
+						return
+					}
+					var agents []codersdk.WorkspaceAgent
+					for _, resource := range resources {
+						agents = append(agents, resource.Agents...)
+					}
+
+					mu.Lock()
+					defer mu.Unlock()
+					if len(agents) == 1 {
+						completions = append(completions, ws.Name)
+					} else {
+						for _, agent := range agents {
+							completions = append(completions, fmt.Sprintf("%s.%s", ws.Name, agent.Name))
+						}
+					}
+				}()
+			}
+			wg.Wait()
+
+			slices.Sort(completions)
+			return completions
+		},
 		Handler: func(inv *serpent.Invocation) (retErr error) {
 			client, err := r.InitClient(inv)
 			if err != nil {