docs(tutorial): treat App Insights as already-connected by default

[placerda]

Why

PO doing the prompt-agent quickstart pointed out that section 8's optional App Insights subsection reads as if the resource might not exist, when in fact step 3 (both Path A and Path B) already instructs the reader to attach App Insights to the dev project:

• Path A (manual portal) — lists 'Attach or create an Application Insights resource for telemetry, starting with the dev project' among the project-creation steps.
• Path B (microsoft-foundry skill) — the prompt explicitly tells the skill to 'Attach or create an Application Insights resource for telemetry, starting with the dev project.'

So the default expectation should be 'it already exists', with a quick verification, and only fall back to manual creation/override if the verification fails.

What changed

Only docs/tutorial-prompt-agent-quickstart.md. Net: +51 / -26 lines.

1. Section 7 callout rewritten to lead with 'should already be wired from step 3' and give a 10-second portal check.
2. Section 8 optional subsection restructured:
   • Opens with 'you can skip this subsection' and a 2-row decision table for the verification outcome.
   • Adds a 'just click Connect Application Insights in the Foundry portal' fallback for the rare case the resource was not created in step 3.
   • Demotes the manual APPLICATIONINSIGHTS_CONNECTION_STRING paste to an 'override which resource telemetry goes to' advanced case at the end.
3. Path A → C portal/CLI instructions remain available for anyone who actually needs the override.

Out of scope

The hosted-agent and end-to-end tutorials have different App Insights flows (wizard-driven, local FastAPI sample needing the env variable to enable OpenTelemetry export) — those are intentionally untouched here. Only the prompt-agent quickstart had the 'might not exist' framing.

Validation

Documentation-only change, no tests required.