Redact credentials on the way into a stored chat

A stored transcript is replayed into a later prompt when reopened, so a key typed while
changing a setting would be handed back to the model on every subsequent turn.

AI/README-AI.md

@@ -299,6 +299,31 @@ Reopened chats render as plain turns: sources, reasoning and timings describe on
 are not stored, because redrawing them beside a transcript that may be continued under a
 different profile would be citing evidence for an answer no longer being made.
 
+### Secrets are redacted on the way to disk
+
+A conversation about settings is a conversation containing credentials — asking for an API key to
+be changed means typing one. Message bodies are redacted in `vv_ai_chat_save()`, and the question
+is redacted again before it reaches `ai.log`.
+
+**On the way out, never in flight.** The live turn keeps the real value, because the model needs
+it to carry out what was asked. What it does not need is that value still in the transcript a week
+later — and a stored chat is replayed into a later prompt when reopened, so an unredacted one
+would hand the credential back on every subsequent turn, indefinitely.
+
+Two passes, because they catch different things:
+
+| Pass | Catches | Method |
+|---|---|---|
+| Known values | a credential this host already holds | exact match against secret-shaped conf keys, longest first |
+| Assignment shapes | a credential arriving that is not in the conf yet | `NAME=value`, `"api_key": value`, "set the token to …" |
+
+The second pass is the one that matters for settings changes: *"change the Emby API key to X"* is
+a secret arriving, and X matches nothing on disk until after the write it is requesting.
+
+Ordinary prose is left alone — the patterns anchor on a secret-shaped *name*, so `CACHE_WARN_GB=100`
+and "turn off the zfs scrub" pass through untouched. `vv_conf_key_is_secret()` is shared with the
+conf audit log, so the two cannot disagree about what counts as a secret.
+
 ## ━━━ TOKEN ACCOUNTING ━━━
 
 Every completed `ask` appends one row to `AI_TOKEN_DB` (`data/ai/ai_token_history.db`):