Add ML and QA Engineer documentation and workflows

- Introduced ML Engineer role with detailed responsibilities, success metrics, and workflow documentation.
- Established QA Engineer role with clear responsibilities, limitations, and success metrics.
- Created structured onboarding files for both roles, including README, ROLE, RESPONSIBILITIES, WORKFLOW, and SUCCESS_METRICS.
- Defined limitations for both roles to clarify boundaries and escalation paths.
- Enhanced security engineer documentation with responsibilities, limitations, and workflow for handling security reviews and findings.

employees/qa/README.md

@@ -0,0 +1,18 @@
+# QA Engineer
+
+Built following the reference pattern established in
+[`../project-manager/`](../project-manager/) — see that folder's `README.md` for why the
+structure looks like this.
+
+## Files, in onboarding order
+
+1. [ROLE.md](ROLE.md) — the one-sentence mission and where this role sits in the org
+2. [RESPONSIBILITIES.md](RESPONSIBILITIES.md) — concrete duties
+3. [LIMITATIONS.md](LIMITATIONS.md) — what this role must never do, and what it must escalate
+4. [WORKFLOW.md](WORKFLOW.md) — this role's specific operational loop
+5. [PROMPT.md](PROMPT.md) — the system prompt used to instantiate this agent
+6. [MEMORY.md](MEMORY.md) — this role's own accumulated, role-specific memory
+7. [SUCCESS_METRICS.md](SUCCESS_METRICS.md) — how this role's performance is actually judged
+
+Read `../../ONBOARDING.md` first — it governs the order role folders get read relative to the
+rest of the repository. This README only governs the order within the folder.