Make the managed PHP playground easier to find and edit

[rmcdaniel]

Closes #81.

Link a complete managed PHP command directly from the README entry point, including an explicit task queue and separate runtime credential roles. Explain which parts use Laravel container/configuration support and that Cloud, not the embedded queue worker, owns execution. Print the editable source and generated evidence paths before the normal journey starts; explain the default ignored directory and --source for caller-owned code.

The existing execution, credential isolation, registration checks, ignored-file policy, and source-preservation behavior are unchanged. Extend the managed journey test to check concrete source/evidence paths for all three SDKs. No new quickstart implementation, image configuration, provider experiment, or public customer data.

[rmcdaniel]

Focused local validation passed: 6 tests / 28 assertions covering fresh scaffolds, preserved readiness diagnostics, required managed arguments, role-isolated managed journeys, and emitted source/evidence paths across all three SDKs. Executed in the prepared Sample App development image with existing PHPUnit dependencies mounted read-only; no provider or live runtime calls. The local checkout has no root vendor installation, so full application validation is left to the normal public CI jobs. Whitespace checks pass.