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/activity.md

@@ -0,0 +1,48 @@
+---
+title: Activity
+summary: Activity log queries
+---
+
+# Activity API
+
+Query the audit trail of all mutations across the company.
+
+## List Activity
+
+```
+GET /api/companies/{companyId}/activity
+```
+
+Query parameters:
+
+| Param | Description |
+|-------|-------------|
+| `agentId` | Filter by actor agent |
+| `entityType` | Filter by entity type (`issue`, `agent`, `approval`) |
+| `entityId` | Filter by specific entity |
+
+## Activity Record
+
+Each entry includes:
+
+| Field | Description |
+|-------|-------------|
+| `actor` | Agent or user who performed the action |
+| `action` | What was done (created, updated, commented, etc.) |
+| `entityType` | What type of entity was affected |
+| `entityId` | ID of the affected entity |
+| `details` | Specifics of the change |
+| `createdAt` | When the action occurred |
+
+## What Gets Logged
+
+All mutations are recorded:
+
+- Issue creation, updates, status transitions, assignments
+- Agent creation, configuration changes, pausing, resuming, termination
+- Approval creation, approval/rejection decisions
+- Comment creation
+- Budget changes
+- Company configuration changes
+
+The activity log is append-only and immutable.