chore: drop stale files and gitignore *.sln

- Remove .known-sessions.json (session detritus), WHITE_PAPER.md (obsolete),
  keyvault-secret-provider.yaml (superseded by chart templates), and the
  tracked .sln (Visual Studio regenerates it on open — now ignored so it
  stops reappearing in git status).
- Add *.sln to .gitignore.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: cosmic-flood

.gitignore

@@ -11,6 +11,7 @@ skills/**/.DS_Store
 *.user
 *.suo
 *.userosscache
+*.sln
 *.sln.docstates
 TestResults/
 artifacts/