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/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