Add --kms-key-arn flag to add memory (parity with harness/import)

## Summary
The memory schema supports a CMK (`encryptionKeyArn`) and the CLI already wires it through `add harness --memory-encryption-key-arn` and `import memory` — but the standalone `add memory` command has no flag, creating a parity gap.

## Evidence
- **AWS:** [`CreateMemory.encryptionKeyArn`](https://docs.aws.amazon.com/bedrock-agentcore-control/latest/APIReference/API_CreateMemory.html).
- **CLI:** schema field at `src/schema/schemas/agentcore-project.ts:278`; wired via `HarnessPrimitive.ts:667` and `import-memory.ts:81`, but **no flag** in `MemoryPrimitive.tsx`.

## Proposal
Add an optional `--kms-key-arn ` flag (no default) to `add memory`, mapping to the existing `encryptionKeyArn` schema field. Use `--kms-key-arn` (not the harness-scoped `--memory-encryption-key-arn`) to match `add dataset`/`add evaluator`/`add policy-engine`.

Start in MemoryPrimitive.tsx and compare its option definitions with the existing wiring in HarnessPrimitive.tsx and import-memory.ts. Check the schema field at src/schema/schemas/agentcore-project.ts:278 and the add dataset, evaluator, and policy-engine flag naming. Done means add memory accepts an optional --kms-key-arn and maps it to encryptionKeyArn without a default.

Written by the indexing model from the issue text.