fix(word-addin): remove the session-revoking signOut from the OAuth dialog

WHY THIS MATTERS
Two independent verification agents auditing the previous commit both
proved the same severe regression: the cleanup call added there -
`supabase.auth.signOut({ scope: "local" })` - is NOT local. In the
pinned @supabase/auth-js, _signOut always calls
admin.signOut(accessToken, scope), which POSTs `/logout?scope=local`
to GoTrue with the session's JWT. GoTrue's handler deletes that
session row and revokes its refresh token server-side - the exact
refresh token the dialog hands to the task pane two lines later.
Google sign-in would have appeared to work, then silently logged the
user out at the first token refresh (~1 hour), with no test able to
catch it because the e2e suite stubs the dialog file out entirely.

WHAT "LOCAL SIGN-OUT" ACTUALLY MEANS IN GOTRUE
scope=local revokes the CURRENT session only (vs. "global" = all of
the user's sessions, "others" = all but this one). "Local" refers to
which sessions are revoked, not to whether a network call happens -
a subtlety the removed comment got exactly backwards.

HOW THE FIX WORKS
Delete the call. It was also redundant: clearTemporaryAuthStorage()
on the next line removes the persisted session keys, and the client
is created with autoRefreshToken: false, so no refresh timer can
re-persist anything. Also removed: the option-less messageParent
retry, which both verifiers flagged as speculative (every supported
host has the DialogOrigin 1.1 set) and mildly harmful - dropping
targetOrigin feeds the task pane's fail-open origin check. A visible
failure message beats a weaker handoff.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RuPCULDYgVsiCanRgbEy6W