feat: add manual chat title regeneration (#23633)

## Summary

Adds a "Generate new title" action that lets users manually regenerate a
chat's title using richer conversation context than the automatic
first-message title path.

## Changes

### Backend
- **New endpoint:** `POST
/api/experimental/chats/{chatID}/title/regenerate` returns the updated
Chat with a regenerated title
- **Manual title algorithm:** Extracts useful user/assistant text turns
→ selects first user turn + last 3 turns → builds context with gap
markers → renders prompt with anti-recency guidance → calls lightweight
model → normalizes output
- **Helpers:** `extractManualTitleTurns`,
`selectManualTitleTurnIndexes`, `buildManualTitleContext`,
`renderManualTitlePrompt`, `generateManualTitle` — all private, with the
public `Server.RegenerateChatTitle` method
- **SDK:** `ExperimentalClient.RegenerateChatTitle(ctx, chatID) (Chat,
error)`
- Persists title via existing `UpdateChatByID` and broadcasts
`ChatEventKindTitleChange`

### Frontend
- API client method + React Query mutation with cache invalidation
- "Generate new title" menu item (with wand icon) in both TopBar and
Sidebar dropdown menus
- Loading/disabled state while regeneration is in-flight
- Error toast on failure
- Stories updated for both menus

### Tests
- `quickgen_test.go`: Table-driven tests for all 4 helper functions
(turn extraction, index selection, context building, prompt rendering)
- `exp_chats_test.go`: Handler tests (ChatNotFound,
NotFoundForDifferentUser, NoDaemon)

## Design notes
- The existing auto-title path (`maybeGenerateChatTitle`, `titleInput`)
is completely unchanged
- Manual regeneration uses richer context (first user turn + last 3
turns + gap markers) vs the auto path's single first message
- Endpoint is experimental and marked with `@x-apidocgen {"skip": true}`

coderd/database/queries/chats.sql

@@ -220,6 +220,21 @@ WHERE
 ORDER BY
     created_at ASC;
 
+-- name: GetChatMessagesByChatIDAscPaginated :many
+SELECT
+    *
+FROM
+    chat_messages
+WHERE
+    chat_id = @chat_id::uuid
+    AND id > @after_id::bigint
+    AND visibility IN ('user', 'both')
+    AND deleted = false
+ORDER BY
+    id ASC
+LIMIT
+    COALESCE(NULLIF(@limit_val::int, 0), 50);
+
 -- name: GetChatMessagesByChatIDDescPaginated :many
 SELECT
     *
@@ -466,6 +481,17 @@ WHERE
 RETURNING
     *;
 
+-- name: UpdateChatLastModelConfigByID :one
+UPDATE
+    chats
+SET
+    -- NOTE: updated_at is intentionally NOT touched here to avoid changing list ordering.
+    last_model_config_id = @last_model_config_id::uuid
+WHERE
+    id = @id::uuid
+RETURNING
+    *;
+
 -- name: UpdateChatLabelsByID :one
 UPDATE
     chats
@@ -550,6 +576,21 @@ WHERE
 RETURNING
     *;
 
+-- name: UpdateChatStatusPreserveUpdatedAt :one
+UPDATE
+    chats
+SET
+    status = @status::chat_status,
+    worker_id = sqlc.narg('worker_id')::uuid,
+    started_at = sqlc.narg('started_at')::timestamptz,
+    heartbeat_at = sqlc.narg('heartbeat_at')::timestamptz,
+    last_error = sqlc.narg('last_error')::text,
+    updated_at = @updated_at::timestamptz
+WHERE
+    id = @id::uuid
+RETURNING
+    *;
+
 -- name: GetStaleChats :many
 -- Find chats that appear stuck (running but heartbeat has expired).
 -- Used for recovery after coderd crashes or long hangs.