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>

agent.yml.example

@@ -26,16 +26,39 @@ public_key: ""
 
 location: "default" # Location identifier (e.g., site, datacenter) for naming
 
+# Local LDAP byte-pump socket (DESIGN.md §4). The agent forwards raw LDAP bytes
+# from this socket to the SSO, which relays them into its OpenLDAP. The agent
+# never parses LDAP. Point SSSD at it with:
+#   ldap_uri = ldapi://%2frun%2ftheta%2fldap.sock
+ldap_socket: "/run/theta/ldap.sock"
+
 capabilities:
   # ---------------------------------------------------------
   # Basic Capabilities (Safe, read-only or infrastructure management)
   # ---------------------------------------------------------
-  
+
   # Push CPU, RAM, GPU, and ZFS metrics to the SSO Manager
   telemetry: true
-  
+
   # Allow the SSO Manager to push down SSSD and SSH keys configuration
   configure_ldap: true
+
+  # Serve the local LDAP byte-pump socket for SSSD/PAM (DESIGN.md §4)
+  ldap_tunnel: true
+
+  # Render OpenBao secrets to local files from /etc/theta/templates (DESIGN.md §5)
+  secrets: true
+
+  # Apply node IAM (sudo rules, SSH keys, access control, revocation) (DESIGN.md §6)
+  iam: true
+
+# Secret templates to render (DESIGN.md §5). Each maps a local template to a
+# target file and an optional post-render reload. The template embeds secrets as
+# {{ bao "secret/data/nodes/<node-id>/<name>#<key>" }}.
+# secrets:
+#   - template: /etc/theta/templates/db.env.tpl
+#     target: /etc/theta/db.env
+#     reload: systemctl reload app
   
   # ---------------------------------------------------------
   # Advanced Capabilities (High risk, remote operations)