Stabilize `verify-graph` Playwright flakiness

[bracyw]

Follow-up from the Matt Pocock skills adoption PR (#646).

The verify-graph skill (Playwright MCP verification of the graph page) can be flaky during real development.

Acceptance

• Harden the steps/selectors in .claude/skills/verify-graph/SKILL.md for mode switching, controls, and topic selection.
• Document any known-flaky modes and recommended waits/retries.
• Verification should run reliably on a clean local stack (run-local).

Tooling-reliability follow-up; not blocking.