Removes OpenAI Responses "chain mode" from chatd. Closes CODAGT-445.
- Deletes `chatopenai/responses.go` (chain detection, activation, prompt filtering, response ID extraction) and its tests.
- Deletes the `ChainBroken` classification in `chaterror` and the chatloop retry bookkeeping that disabled chain mode mid-generation.
- Drops the `chain_broken` label from the `coderd_chatd_stream_retries_total` metric.
- Stops reading and writing `chat_messages.provider_response_id`
- Deletes the dead `ClearChatMessageProviderResponseIDsByChatID` query. Dropping the column is a follow-up migration.
- Deletes three chatloop hooks no caller sets (`ReloadMessages`, `DisableChainMode`, `PrepareMessages`), the dead `const AgentChatContextSentinelPath`, and stale chain-mode comments.
🤖 Generated by Coder Agents on behalf of @johnstcn.
Addresses
[CODAGT-620](https://linear.app/codercom/issue/CODAGT-620/session-can-get-stuck-at-compaction-with-request-failed).
We have logic that checks whether message compaction still leaves the
chat over the context limit. We want to abort if it does - if we didn't,
we'd get into an endless compaction loop. The check's logic was faulty.
This PR changes fixes it. The new flow is:
1. In iteration 1, a chat runner commits a message compaction summary.
2. In iteration 2, the runner submits the newly compacted conversation
to the LLM provider in order to generate the next message.
3. In iteration 3, 4, 5, etc., if the conversation needs compaction, the
runner looks up the configured context limit and the first assistant
message after the last compaction summary. It compares the context usage
on that message with the context limit. If the usage is over the limit,
it returns an error.
Cian Johnston