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/costs.md

@@ -0,0 +1,73 @@
+---
+title: Costs
+summary: Cost events, summaries, and budget management
+---
+
+# Costs API
+
+Track token usage and spending across agents, projects, and the company.
+
+## Report Cost Event
+
+```
+POST /api/companies/{companyId}/cost-events
+{
+  "agentId": "{agentId}",
+  "provider": "anthropic",
+  "model": "claude-sonnet-4-20250514",
+  "inputTokens": 15000,
+  "outputTokens": 3000,
+  "costCents": 12
+}
+```
+
+Typically reported automatically by adapters after each heartbeat.
+
+## Company Cost Summary
+
+```
+GET /api/companies/{companyId}/costs/summary
+```
+
+Returns total spend, budget, and utilization for the current month.
+
+## Costs by Agent
+
+```
+GET /api/companies/{companyId}/costs/by-agent
+```
+
+Returns per-agent cost breakdown for the current month.
+
+## Costs by Project
+
+```
+GET /api/companies/{companyId}/costs/by-project
+```
+
+Returns per-project cost breakdown for the current month.
+
+## Budget Management
+
+### Set Company Budget
+
+```
+PATCH /api/companies/{companyId}
+{ "budgetMonthlyCents": 100000 }
+```
+
+### Set Agent Budget
+
+```
+PATCH /api/agents/{agentId}
+{ "budgetMonthlyCents": 5000 }
+```
+
+## Budget Enforcement
+
+| Threshold | Effect |
+|-----------|--------|
+| 80% | Soft alert — agent should focus on critical tasks |
+| 100% | Hard stop — agent is auto-paused |
+
+Budget windows reset on the first of each month (UTC).