feat(tools/git): explain tea's misleading user does not exist error

`user does not exist [uid: 0, name: ]` from tea reads as a missing account.
It almost always means a revoked or stale token: `tea login` keeps its own
copy of the token, so rotating the credential store does not update it. The
error points at the account; the cause is the cached credential.

Adds explain_tea_user_does_not_exist() to detect-platform.sh and invokes it
from the three tea-failure fallback paths, each guarded by `declare -F` so
the call site is independent of load order and a no-op if the helper is absent.

Purely additive: stderr only, no control flow, no gate or credential changes.

Verified both directions: the helper emits when sourced, and the guarded call
is a silent no-op (exit 1, no output) when it is not.

Refs #1082

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Amf1Neca162odgcbCWMk1y

packages/mosaic/framework/tools/git/pr-create.sh

@@ -219,6 +219,7 @@ case "$PLATFORM" in
             exit 0
         fi
         echo "Warning: tea pr create failed, trying Gitea API fallback..." >&2
+        declare -F explain_tea_user_does_not_exist >/dev/null && explain_tea_user_does_not_exist
         gitea_pr_create_api
         ;;
     *)