[Agentic Chat] Create custom rules - first iteration

Custom rules initial iteration

As a part of this issue, we implemented a first iteration of custom rules which:

• looks in the project's .gitlab/rules/chat.md file
• attaches its content as user-provided rules for each agentic chat session

MRs

• Add custom user rules PU for Duo (chat, workflo... (gitlab-org/cloud-connector/gitlab-cloud-connector!167 - merged)
• feat: Duo Agentic Chat - user provided rules (gitlab-org/editor-extensions/gitlab-lsp!1878 - merged)

Full epic: &16938