Coder Agents chats could get stuck showing "Thinking" forever when a
title regenerate/propose request ran while a generation was in flight.
Manual title generation recorded token cost by inserting a hidden
assistant message into `chat_messages` and immediately soft-deleting it.
Triggers on that table sync `chats.history_version` to
`snapshot_version`, so this out-of-band write broke the
`history_version` fence of an in-flight generation task, killing it
without a replacement and leaving the chat stuck in `running`.
Remove the accounting path entirely; AI Gateway already records
title-call usage in `aibridge_interceptions`/`aibridge_token_usages`.
The manual title endpoints no longer write to `chat_messages` at all,
and new regression tests assert `history_version` stays untouched. Note
this intentionally drops title-generation cost from chatd's chat-level
cost surfaces; it still counts against the user's AI budget via AI
Gateway.
Closes CODAGT-595
Closes
[CODAGT-610](https://linear.app/codercom/issue/CODAGT-610/add-an-architecturemd-file-to-chatd).
Adds an ARCHITECTURE.md file which describes the architecture of the
chatd subsystem. It's meant for reading by both humans, who would like
to understand chatd better, and agents. It's an edited version of the
chatd stabilization RFC.
Ethan