fix: update the compaction message to be the "user" role (#22819)

## Bug

After compaction in the chat loop, the loop re-enters and calls the LLM
with a prompt that has **no non-system messages**. Anthropic (and most
providers) require at least one user/assistant/tool message, so the API
errors with empty messages.

## Root Cause

The compaction summary was stored as `role=system`. After compaction,
`GetChatMessagesForPromptByChatID` returns only:
- The compressed system summary (matched by the CTE)
- Original non-compressed system messages (system prompts)

All original user/assistant/tool messages are excluded (they predate the
summary). The compaction assistant/tool messages are `compressed=TRUE`
and don't match the main query's `compressed=FALSE` clauses.

So `ReloadMessages` returned only system messages. The Anthropic
provider moves system messages into a separate `system` field, leaving
the `messages` API field as `[]`.

## Fix

1. **Changed compaction summary from `role=system` to `role=user`** —
the summary now appears as a user message in the reloaded prompt, giving
the model valid conversational context to respond to.

2. **Simplified the CTE** — removed the `role = 'system'` check and
narrowed `visibility IN ('model', 'both')` to just `visibility =
'model'`. The summary is the only compressed message with
`visibility=model` (the assistant has `visibility=user`, the tool has
`visibility=both`), so the role check was redundant.

## Test

`PostRunCompactionReEntryIncludesUserSummary`: verifies the re-entry
prompt contains a user message (the compaction summary) after compaction
+ reload.

coderd/database/queries.sql.go

@@ -3321,9 +3321,8 @@ WITH latest_compressed_summary AS (
         chat_messages
     WHERE
         chat_id = $1::uuid
-        AND role = 'system'
-        AND visibility IN ('model', 'both')
         AND compressed = TRUE
+        AND visibility = 'model'
     ORDER BY
         created_at DESC,
         id DESC