docs: name Invoke-Build's .build.ps1 default in migration prompt

The AI-assisted migration prompt listed only ./build.ps1 as the default
consumer build file. Invoke-Build users conventionally use ./.build.ps1
(as the repo's own README example does), so name both defaults to reduce
agent back-and-forth. Addresses Copilot review feedback on #123.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015cqAkNY5JkcoroyEAmbeVj

Author: tablackburn

docs/migration-v0.8-to-v1.0.md

@@ -31,8 +31,8 @@ Inputs:
 - This migration guide: docs/migration-v0.8-to-v1.0.md in the
   psake/PowerShellBuild repository on GitHub. Fetch and read it if you
   have web or repo access; otherwise ask me to paste it.
-- My build file (default: ./build.ps1; ask if it lives elsewhere or has
-  a different name).
+- My build file (default: ./build.ps1 for psake, or ./.build.ps1 for
+  Invoke-Build; ask if it lives elsewhere or has a different name).
 - Any psake or Invoke-Build files my build file references.
 
 Task: