feat: add boundary log forwarding from agent to coderd (#21345)

Add agent forwarding of boundary audit logs from workspaces to coderd
via agent API, and re-emission of boundary logs to coderd stderr. This
change adds a server to the workspace agent that always listens on a
unix socket for boundary to connect and send audit logs.

coderd log format example:
```
[API] 2025-12-23 18:31:46.755 [info] coderd.agentrpc: boundary_request owner=.. workspace_name=.. agent_name=.. decision=.. workspace_id=.. http_method=.. http_url=.. event_time=.. request_id=..
```

Corresponding boundary PR: https://github.com/coder/boundary/pull/124
RFC:
https://www.notion.so/coderhq/Agent-Boundary-Logs-2afd579be59280f29629fc9823ac41ba
https://github.com/coder/coder/issues/21280

cli/agent.go

@@ -31,6 +31,7 @@ import (
 	"github.com/coder/coder/v2/agent/agentcontainers"
 	"github.com/coder/coder/v2/agent/agentexec"
 	"github.com/coder/coder/v2/agent/agentssh"
+	"github.com/coder/coder/v2/agent/boundarylogproxy"
 	"github.com/coder/coder/v2/agent/reaper"
 	"github.com/coder/coder/v2/buildinfo"
 	"github.com/coder/coder/v2/cli/clilog"
@@ -59,6 +60,7 @@ func workspaceAgent() *serpent.Command {
 		devcontainerDiscoveryAutostart bool
 		socketServerEnabled            bool
 		socketPath                     string
+		boundaryLogProxySocketPath     string
 	)
 	agentAuth := &AgentAuth{}
 	cmd := &serpent.Command{
@@ -319,8 +321,9 @@ func workspaceAgent() *serpent.Command {
 						agentcontainers.WithProjectDiscovery(devcontainerProjectDiscovery),
 						agentcontainers.WithDiscoveryAutostart(devcontainerDiscoveryAutostart),
 					},
-					SocketPath:          socketPath,
-					SocketServerEnabled: socketServerEnabled,
+					SocketPath:                 socketPath,
+					SocketServerEnabled:        socketServerEnabled,
+					BoundaryLogProxySocketPath: boundaryLogProxySocketPath,
 				})
 
 				if debugAddress != "" {
@@ -494,6 +497,13 @@ func workspaceAgent() *serpent.Command {
 			Description: "Specify the path for the agent socket.",
 			Value:       serpent.StringOf(&socketPath),
 		},
+		{
+			Flag:        "boundary-log-proxy-socket-path",
+			Default:     boundarylogproxy.DefaultSocketPath(),
+			Env:         "CODER_AGENT_BOUNDARY_LOG_PROXY_SOCKET_PATH",
+			Description: "The path for the boundary log proxy server Unix socket. Boundary should write audit logs to this socket.",
+			Value:       serpent.StringOf(&boundaryLogProxySocketPath),
+		},
 	}
 	agentAuth.AttachOptions(cmd, false)
 	return cmd