A wiki can hold the working instructions your agents follow, including those several projects share: a page per project or topic, and one page of common guidance that each of them links to. A wiki's own rules are what the server hands every agent before it writes there.
Inside a project, a short AGENTS.md or CLAUDE.md can point to those pages, so the instructions grow and change in one place. An agent with access to the wiki reads them through MCP when it starts a task; its client still loads its local instruction files as usual.