fix(docs): correct session timeout and concurrency defaults in COST_MODEL

- Session timeout: 8 hours → 9 hours (matches task-orchestrator.ts:173)
- Concurrency limit: 2 → 3 (matches task-orchestrator.ts:163 default)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: scottschreckengaust

docs/design/COST_MODEL.md

@@ -76,8 +76,8 @@ For multi-user deployments, cost should be attributable to individual users and
 |---|---|---|
 | Turn limit | `max_turns` per task | 100 |
 | Cost budget | `max_budget_usd` per task | None (unlimited) |
-| Session timeout | Orchestrator timeout | 8 hours |
-| Concurrency limit | Per-user atomic counter | 2 concurrent tasks |
+| Session timeout | Orchestrator timeout | 9 hours |
+| Concurrency limit | Per-user atomic counter | 3 concurrent tasks |
 | System concurrency | System-wide counter | Account-level AgentCore quota |
 
 ## Additional guardrails

docs/src/content/docs/architecture/Cost-model.md

@@ -80,8 +80,8 @@ For multi-user deployments, cost should be attributable to individual users and
 |---|---|---|
 | Turn limit | `max_turns` per task | 100 |
 | Cost budget | `max_budget_usd` per task | None (unlimited) |
-| Session timeout | Orchestrator timeout | 8 hours |
-| Concurrency limit | Per-user atomic counter | 2 concurrent tasks |
+| Session timeout | Orchestrator timeout | 9 hours |
+| Concurrency limit | Per-user atomic counter | 3 concurrent tasks |
 | System concurrency | System-wide counter | Account-level AgentCore quota |
 
 ## Additional guardrails