docs: add external documentation site content

Add structured documentation covering quickstart, architecture, core
concepts, API reference, adapter guides, CLI commands, deployment
options, and operator/developer guides.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/api/secrets.md

@@ -0,0 +1,57 @@
+---
+title: Secrets
+summary: Secrets CRUD
+---
+
+# Secrets API
+
+Manage encrypted secrets that agents reference in their environment configuration.
+
+## List Secrets
+
+```
+GET /api/companies/{companyId}/secrets
+```
+
+Returns secret metadata (not decrypted values).
+
+## Create Secret
+
+```
+POST /api/companies/{companyId}/secrets
+{
+  "name": "anthropic-api-key",
+  "value": "sk-ant-..."
+}
+```
+
+The value is encrypted at rest. Only the secret ID and metadata are returned.
+
+## Update Secret
+
+```
+PATCH /api/secrets/{secretId}
+{
+  "value": "sk-ant-new-value..."
+}
+```
+
+Creates a new version of the secret. Agents referencing `"version": "latest"` automatically get the new value on next heartbeat.
+
+## Using Secrets in Agent Config
+
+Reference secrets in agent adapter config instead of inline values:
+
+```json
+{
+  "env": {
+    "ANTHROPIC_API_KEY": {
+      "type": "secret_ref",
+      "secretId": "{secretId}",
+      "version": "latest"
+    }
+  }
+}
+```
+
+The server resolves and decrypts secret references at runtime, injecting the real value into the agent process environment.