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>

demo/Dockerfile

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+FROM node:20-alpine
+RUN apk add --no-cache bash
+WORKDIR /demo
+COPY get-secret.sh get-secret.js ./
+RUN chmod +x get-secret.sh
+CMD ["sh", "-c", "sh /demo/get-secret.sh && node /demo/get-secret.js"]

demo/get-secret.js

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+// Demo: a 3rd-party node app reads the secret the theta-agent rendered to disk.
+const fs = require('fs');
+const path = '/etc/theta/rendered/db.env';
+console.log('=== node app reads the rendered secret ===');
+if (fs.existsSync(path)) {
+  const env = fs.readFileSync(path, 'utf8');
+  const db = {};
+  for (const line of env.split('\n')) {
+    const m = /^(\w+)="(.*)"$/.exec(line.trim());
+    if (m) db[m[1]] = m[2];
+  }
+  console.log('DB_USER=' + db.DB_USER);
+  console.log('DB_PASS=' + db.DB_PASS);
+} else {
+  console.error('rendered secret not found — run render_secrets first');
+  process.exit(1);
+}

demo/get-secret.sh

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+#!/bin/sh
+# Demo: a 3rd-party bash app reads the secret the theta-agent rendered to disk.
+# The agent rendered /etc/theta/rendered/db.env from a template + OpenBao.
+echo "=== bash app reads the rendered secret ==="
+if [ -f /etc/theta/rendered/db.env ]; then
+  . /etc/theta/rendered/db.env
+  echo "DB_USER=$DB_USER"
+  echo "DB_PASS=$DB_PASS"
+else
+  echo "rendered secret not found — run render_secrets first"
+  exit 1
+fi