Mem-Client: Test Selector Convention

Mem-client component markup must use `data-test-id` selector convention.

Rationale:
- A single selector convention improves test consistency and migration hygiene.

Scope:
- PR review policy for component markup changes under `domains/frontend/mem-client/src/**`.
- Excludes tests/stories.

Trigger:
- A PR adds new test selector attributes in scoped component markup.

Approval checks:
1) New selector attributes use `data-test-id`.
2) New `data-testid` attributes are not introduced in scoped runtime components.

Evidence:
- Diff of component markup shows selector attributes follow convention.

Decision:
- Return NOT APPROVED when trigger is met and `data-testid` is introduced.