Improve the error message for missing action scan paths

[Metbcy]

When a user sets the Action's path: input to a directory that does not exist in one checkout, the current failure is correct but can be more actionable.

Scope:

• Improve the scan path not found message in entrypoint.sh with examples for monorepos and non-PR events.
• Keep the failure loud; do not silently fall back to repository root.
• Add or update a shell-level validation note in docs/src/github-action.md.

Definition of done:

• bash -n entrypoint.sh passes.
• The docs explain how to fix the error.