feat: add --ssh-host-prefix flag for "coder ssh" (#16088)

This adds a flag matching `--ssh-host-prefix` from `coder config-ssh` to
`coder ssh`. By trimming a custom prefix from the argument, we can set
up wildcard-based `Host` entries in SSH config for the IDE plugins (and
eventually `coder config-ssh`).

We also replace `--` in the argument with `/`, so ownership can be
specified in wildcard-based SSH hosts like `<owner>--<workspace>`.

Replaces #16087.

Part of https://github.com/coder/coder/issues/14986.

Related to https://github.com/coder/coder/pull/16078 and
https://github.com/coder/coder/pull/16080.

cli/ssh.go

@@ -61,6 +61,7 @@ var (
 func (r *RootCmd) ssh() *serpent.Command {
 	var (
 		stdio               bool
+		hostPrefix          string
 		forwardAgent        bool
 		forwardGPG          bool
 		identityAgent       string
@@ -195,7 +196,11 @@ func (r *RootCmd) ssh() *serpent.Command {
 				parsedEnv = append(parsedEnv, [2]string{k, v})
 			}
 
-			workspace, workspaceAgent, err := getWorkspaceAndAgent(ctx, inv, client, !disableAutostart, inv.Args[0])
+			namedWorkspace := strings.TrimPrefix(inv.Args[0], hostPrefix)
+			// Support "--" as a delimiter between owner and workspace name
+			namedWorkspace = strings.Replace(namedWorkspace, "--", "/", 1)
+
+			workspace, workspaceAgent, err := getWorkspaceAndAgent(ctx, inv, client, !disableAutostart, namedWorkspace)
 			if err != nil {
 				return err
 			}
@@ -509,6 +514,12 @@ func (r *RootCmd) ssh() *serpent.Command {
 			Description: "Specifies whether to emit SSH output over stdin/stdout.",
 			Value:       serpent.BoolOf(&stdio),
 		},
+		{
+			Flag:        "ssh-host-prefix",
+			Env:         "CODER_SSH_SSH_HOST_PREFIX",
+			Description: "Strip this prefix from the provided hostname to determine the workspace name. This is useful when used as part of an OpenSSH proxy command.",
+			Value:       serpent.StringOf(&hostPrefix),
+		},
 		{
 			Flag:          "forward-agent",
 			FlagShorthand: "A",