feat(gateway): session management knobs (timeout, max sessions)

### Description

AgentCore Gateway has session semantics on the data plane (each MCP client establishes a session), but `AWS::BedrockAgentCore::Gateway` does not expose any `SessionTimeout`, `MaxSessions`, or `SessionConfiguration` property. The full Gateway property list is `AuthorizerConfiguration`, `AuthorizerType`, `Description`, `ExceptionLevel`, `InterceptorConfigurations`, `KmsKeyArn`, `Name`, `PolicyEngineConfiguration`, `ProtocolConfiguration`, `ProtocolType`, `RoleArn`, `Tags` — nothing session-related ([CFN docs](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-bedrockagentcore-gateway.html)).

The CLI also has no session knobs on `AgentCoreGatewaySchema` (`src/schema/schemas/mcp.ts`). Customers in long-running multi-tool sessions cannot set idle timeouts or max session counts.

### Acceptance Criteria

- [ ] Confirm with the AgentCore service team which session knobs (if any) are configurable today via the control-plane `CreateGateway` / `UpdateGateway` APIs.
- [ ] If knobs exist: extend `AgentCoreGatewaySchema` with a `sessionConfiguration` block (`sessionTimeoutSeconds?: number`, `maxConcurrentSessions?: number`, etc.) and apply them via imperative calls during deploy (same pattern used for things CFN doesn't yet support).
- [ ] Add `--session-timeout-seconds` and `--max-sessions` flags (or the actual subset of knobs supported).
- [ ] Surface session config in `agentcore status` and in the TUI wizard.
- [ ] When CFN catches up, deprecate the imperative path in favor of CFN.

### Additional Context

- CFN support: **NO** — Gateway has no session-related properties as of 2026-05-21.
- Control-plane: needs service-team confirmation on which session knobs are exposed; may currently be backend-only/non-configurable.
- This may collapse to "no-op until service exposes knobs" — but the schema slot should still be reserved so we don't break compatibility later.