docs: name the ceiling in update_message's keep-on-omit convention

The convention was documented but not its consequence: because None means
keep, no caller can write NULL through this method, and that is invisible
from the signature. Unreachable today since every field is filled once
when a reply finishes and never cleared.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/docs/superpowers/plans/2026-08-09-external-providers.md b/docs/superpowers/plans/2026-08-09-external-providers.md
index d65d6b8..2b448a5 100644
--- a/docs/superpowers/plans/2026-08-09-external-providers.md
+++ b/docs/superpowers/plans/2026-08-09-external-providers.md
@@ -1980,7 +1980,13 @@ Replace `update_message` with:
         completion_tokens: int | None = None,
         model: str | None = None,
     ) -> None:
-        """Fill in a streamed reply. Omitted fields keep their stored value."""
+        """Fill in a streamed reply. Omitted fields keep their stored value.
+
+        ponytail: no caller can write NULL, since None means "keep". No
+        caller needs to: every field here is filled once when a reply
+        finishes and never cleared. The upgrade path if one ever does is a
+        sentinel object distinct from None, not a second method.
+        """
         columns = ["content = ?"]
         values: list = [content]
         for column, value in (