feat(gateway): add gateway-rule primitive for CreateGatewayRule / Update / Delete

### Description

The AgentCore control plane exposes a full GatewayRule API (`CreateGatewayRule`, `UpdateGatewayRule`, `DeleteGatewayRule`, `GetGatewayRule`, `ListGatewayRules` — see [bedrock-agentcore-control APIs](https://docs.aws.amazon.com/bedrock-agentcore-control/latest/APIReference/API_Operations.html)), but there is no `AWS::BedrockAgentCore::GatewayRule` CFN resource and no CLI primitive for it.

GatewayRules let customers express per-target / per-tool routing logic (e.g. route `tool/foo` to target A under condition X, target B otherwise). Today CLI customers must drop into raw AWS SDK calls or the console to manage them — there's no `agentcore add gateway-rule` or `agentcore.json` slot.

### Acceptance Criteria

- [ ] Confirm with the service team that GatewayRule is GA / not preview-gated for CFN.
- [ ] Add a new `GatewayRulePrimitive` (extends `BasePrimitive`) under `src/cli/primitives/`.
- [ ] Add `agentcore add gateway-rule` and `agentcore remove gateway-rule` commands wired into `cli.ts` and the `add/remove` types.
- [ ] Define `AgentCoreGatewayRuleSchema` in `src/schema/schemas/mcp.ts` (or a new `gateway-rule.ts`) with the same fields the control-plane API takes (rule name, target reference, match conditions, priority, etc. — finalized once we have the API shape).
- [ ] Since CFN does not expose GatewayRule yet, apply the rules via control-plane API calls in a post-deploy step (same pattern proposed in #542 for resource-based policies).
- [ ] TUI screens for add/list/remove gateway rules.
- [ ] When CFN ships `AWS::BedrockAgentCore::GatewayRule`, switch to the CDK path and deprecate the imperative one.

### Additional Context

- CFN support: **NO** — no `AWS::BedrockAgentCore::GatewayRule` resource as of 2026-05-21.
- Control-plane: **YES** — full CRUD via `CreateGatewayRule` / `UpdateGatewayRule` / `DeleteGatewayRule` / `GetGatewayRule` / `ListGatewayRules`.
- Pattern: imperative post-deploy calls — the same approach #542 (resource-based policies) is taking.
- Coordinate with: `aws/agentcore-l3-cdk-constructs` (no construct exists yet — once CFN ships, add an L3 wrapper).