Add LDAP byte-pump tunnel, secrets rendering, and IAM engine

See CHANGELOG.md for the full breakdown. Summary:

- ldap_tunnel.go: serves a local unix socket for SSSD/PAM and relays raw
  bytes to the SSO over the existing WSS channel (ldap_tunnel messages);
  the agent never parses LDAP (DESIGN.md §4). Adds safeWriter to
  serialize WebSocket writes now that telemetry, heartbeat, the LDAP
  tunnel, and command responses all share one connection.
- secrets.go: renders local templates ({{ bao "path#key" }} placeholders)
  by fetching node-scoped values from the SSO and writing the target
  atomically at 0600, on a signed render_secrets command (DESIGN.md §5).
  demo/ has minimal bash + Node consumers of the rendered file.
- iam.go: applies signed node IAM pushes -- sudoers.d rules (visudo -c
  validated), SSH AuthorizedKeysCommand keys, /etc/security/access.conf,
  and revocation via sss_cache -E + pkill -u (DESIGN.md §6).
- Capability reporting: the agent's enabled capabilities ride along in
  its discovery frame so the SSO can show them in the Directory.
- DESIGN.md: the v2 protocol design this implements.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

websocket_test.go

@@ -249,7 +249,7 @@ func TestHandleCommand(t *testing.T) {
 			if tc.signed {
 				msg.Payload = sign(t, msg.Payload)
 			}
-			handleCommand(cm, msg, mockConn, mockExec)
+			handleCommand(cm, msg, mockConn, mockExec, nil)
 
 			if tc.expectedNoResponse {
 				if len(mockConn.Messages) != 0 {