feat(agent): make PXI quick actions dynamic per page context (#13534)

* feat(agent): make PXI quick actions dynamic per page context

PXI's empty-state quick actions were hardcoded and identical on every
page. Derive them from the agent contexts PXI already advertises for the
active route (the same signal that gates server-side contextual tools),
so suggestions track what the assistant can do where the user is — e.g.
enhance/run the prompt on the playground, triage/filter on a project.

Closes #13153

* docs(agent): refer to the assistant (not "PXI") in code comments

"PXI" is just the brand name for the assistant; code prose should say
"the assistant". Updates the new quick-action comments and records the
rule in the phoenix-pxi skill.