feat: add coder exp sync list command (#26443)

Add a new subcommand to list all registered sync units and their current
statuses. This provides a quick overview of the dependency coordination
state in a workspace without needing to query each unit individually.

The command supports both table (default) and JSON output formats.

```
$ coder exp sync list
UNIT    STATUS     READY
unit-a  started    true
unit-b  completed  true
unit-c  pending    false

$ coder exp sync list --output json
[
  {
    "unit_name": "my-unit",
    "status": "started",
    "is_ready": true
  }
]
```

When no units are registered, the command prints `No units registered`.

<details><summary>Changes across layers</summary>

- `agent/unit`: add `Manager.ListUnits()` method
- `agent/agentsocket/proto`: add `SyncList` RPC, bump API to v1.2
- `agent/agentsocket`: add service and client implementations
- `cli`: add `sync_list.go` command, register in `sync.go`
- Tests: three golden-file test cases (empty list, multiple units, JSON)

</details>

> Generated by Coder Agents on behalf of @SasSwart

---------

Co-authored-by: Cian Johnston <cian@coder.com>

cli/sync_status.go

@@ -36,7 +36,7 @@ func (*RootCmd) syncStatus(socketPath *string) *serpent.Command {
 	cmd := &serpent.Command{
 		Use:   "status <unit>",
 		Short: "Show unit status and dependency state",
-		Long:  "Show the current status of a unit, whether it is ready to start, and lists its dependencies. Shows which dependencies are satisfied and which are still pending. Supports multiple output formats.",
+		Long:  "Show the current status of a unit, whether it is ready to start, and lists its dependencies. Shows which dependencies are satisfied and which are still pending.",
 		Handler: func(i *serpent.Invocation) error {
 			ctx := i.Context()