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/testdata/TestSyncCommands_Golden/list_json_format.golden

@@ -0,0 +1,7 @@
+[
+  {
+    "unit_name": "my-unit",
+    "status": "started",
+    "is_ready": true
+  }
+]

cli/testdata/TestSyncCommands_Golden/list_no_units.golden

@@ -0,0 +1 @@
+No units registered

cli/testdata/TestSyncCommands_Golden/list_with_units.golden

@@ -0,0 +1,4 @@
+UNIT    STATUS     READY  
+unit-a  started    true   
+unit-b  completed  true   
+unit-c  pending    false  

cli/testdata/coder_exp_sync_--help.golden

@@ -13,6 +13,7 @@ USAGE:
 
 SUBCOMMANDS:
     complete    Mark a unit as complete
+    list        List all registered units and their statuses
     ping        Test agent socket connectivity and health
     start       Wait until all unit dependencies are satisfied
     status      Show unit status and dependency state

cli/testdata/coder_exp_sync_list_--help.golden

@@ -0,0 +1,19 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder exp sync list [flags]
+
+  List all registered units and their statuses
+
+  List all units currently registered with the workspace agent. Shows each
+  unit's name, status, and whether it is ready to start.
+
+OPTIONS:
+  -c, --column [unit|status|ready] (default: unit,status,ready)
+          Columns to display in table output.
+
+  -o, --output table|json (default: table)
+          Output format.
+
+———
+Run `coder --help` for a list of global options.

cli/testdata/coder_exp_sync_status_--help.golden

@@ -7,7 +7,7 @@ USAGE:
 
   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.
+  pending.
 
 OPTIONS:
   -c, --column [depends on|required status|current status|satisfied] (default: depends on,required status,current status,satisfied)