feat: add pr, repo, pr_title chat search filters (#25569)

Relates to CODAGT-432

Adds three new search filters to the chat list endpoint (`GET
/api/experimental/chats/`):

- `pr:<number>` - exact PR number match
- `repo:<owner/repo>` - substring match against git remote origin or URL
- `pr_title:<text>` - case-insensitive PR title substring match

Includes SQL filter clauses (EXISTS against `chat_diff_statuses`),
parser with validation, handler wiring, unit tests, swagger annotation
update, and a new search syntax documentation page.

> 🤖 Generated with [Coder Agents](https://coder.com/agents)

coderd/database/modelqueries.go

@@ -778,6 +778,9 @@ func (q *sqlQuerier) GetAuthorizedChats(ctx context.Context, arg GetChatsParams,
 		arg.TitleQuery,
 		arg.HasUnread,
 		pq.Array(arg.PullRequestStatuses),
+		arg.PrNumber,
+		arg.RepoQuery,
+		arg.PrTitleQuery,
 		arg.OffsetOpt,
 		arg.LimitOpt,
 	)