feat: support disabling reverse/local port forwarding in agent SSH server (#24026)

The agent SSH server unconditionally allows all four SSH forwarding
paths (TCP local, TCP reverse, Unix local, Unix reverse). This is a
sandbox escape vector when workspaces are used for AI agent containment
— a reverse tunnel lets anything inside the workspace reach the user's
local machine, bypassing network isolation.

This adds two new agent CLI flags / environment variables:

- `--block-reverse-port-forwarding` /
`CODER_AGENT_BLOCK_REVERSE_PORT_FORWARDING` — blocks both TCP (`ssh -R`)
and Unix socket reverse forwarding
- `--block-local-port-forwarding` /
`CODER_AGENT_BLOCK_LOCAL_PORT_FORWARDING` — blocks both TCP (`ssh -L`)
and Unix socket local forwarding

Template admins can set these via the `env` block on the container/VM
resource that runs the agent (e.g. `docker_container`,
`kubernetes_pod`), or via `coder_env` resources tied to the agent.

Fixes https://github.com/coder/coder/issues/22275

<details>
<summary>Implementation notes</summary>

Follows the existing `BlockFileTransfer` pattern:

1. `agent/agentssh/agentssh.go` — New `BlockReversePortForwarding` and
`BlockLocalPortForwarding` fields on `Config`. TCP callbacks check these
before allowing forwarding. The `direct-streamlocal@openssh.com` channel
handler is wrapped to reject Unix local forwards.
2. `agent/agentssh/forward.go` — `forwardedUnixHandler` gains a
`blockReversePortForwarding` field to reject
`streamlocal-forward@openssh.com` requests.
3. `agent/agent.go` — New fields on `Options` and `agent` struct,
plumbed to SSH config.
4. `cli/agent.go` — New serpent flags with env vars.
5. Tests cover all four blocked paths: TCP local, TCP reverse, Unix
local, Unix reverse.

</details>

> 🤖 Generated by Coder Agents

cli/agent.go

@@ -53,6 +53,8 @@ func workspaceAgent() *serpent.Command {
 		slogJSONPath                   string
 		slogStackdriverPath            string
 		blockFileTransfer              bool
+		blockReversePortForwarding     bool
+		blockLocalPortForwarding       bool
 		agentHeaderCommand             string
 		agentHeader                    []string
 		devcontainers                  bool
@@ -319,10 +321,12 @@ func workspaceAgent() *serpent.Command {
 					SSHMaxTimeout:        sshMaxTimeout,
 					Subsystems:           subsystems,
 
-					PrometheusRegistry: prometheusRegistry,
-					BlockFileTransfer:  blockFileTransfer,
-					Execer:             execer,
-					Devcontainers:      devcontainers,
+					PrometheusRegistry:         prometheusRegistry,
+					BlockFileTransfer:          blockFileTransfer,
+					BlockReversePortForwarding: blockReversePortForwarding,
+					BlockLocalPortForwarding:   blockLocalPortForwarding,
+					Execer:                     execer,
+					Devcontainers:              devcontainers,
 					DevcontainerAPIOptions: []agentcontainers.Option{
 						agentcontainers.WithSubAgentURL(agentAuth.agentURL.String()),
 						agentcontainers.WithProjectDiscovery(devcontainerProjectDiscovery),
@@ -493,6 +497,20 @@ func workspaceAgent() *serpent.Command {
 			Description: fmt.Sprintf("Block file transfer using known applications: %s.", strings.Join(agentssh.BlockedFileTransferCommands, ",")),
 			Value:       serpent.BoolOf(&blockFileTransfer),
 		},
+		{
+			Flag:        "block-reverse-port-forwarding",
+			Default:     "false",
+			Env:         "CODER_AGENT_BLOCK_REVERSE_PORT_FORWARDING",
+			Description: "Block reverse port forwarding through the SSH server (ssh -R).",
+			Value:       serpent.BoolOf(&blockReversePortForwarding),
+		},
+		{
+			Flag:        "block-local-port-forwarding",
+			Default:     "false",
+			Env:         "CODER_AGENT_BLOCK_LOCAL_PORT_FORWARDING",
+			Description: "Block local port forwarding through the SSH server (ssh -L).",
+			Value:       serpent.BoolOf(&blockLocalPortForwarding),
+		},
 		{
 			Flag:        "devcontainers-enable",
 			Default:     "true",

cli/testdata/coder_agent_--help.golden

@@ -39,6 +39,12 @@ OPTIONS:
       --block-file-transfer bool, $CODER_AGENT_BLOCK_FILE_TRANSFER (default: false)
           Block file transfer using known applications: nc,rsync,scp,sftp.
 
+      --block-local-port-forwarding bool, $CODER_AGENT_BLOCK_LOCAL_PORT_FORWARDING (default: false)
+          Block local port forwarding through the SSH server (ssh -L).
+
+      --block-reverse-port-forwarding bool, $CODER_AGENT_BLOCK_REVERSE_PORT_FORWARDING (default: false)
+          Block reverse port forwarding through the SSH server (ssh -R).
+
       --boundary-log-proxy-socket-path string, $CODER_AGENT_BOUNDARY_LOG_PROXY_SOCKET_PATH (default: /tmp/boundary-audit.sock)
           The path for the boundary log proxy server Unix socket. Boundary
           should write audit logs to this socket.