Allow administrators to add per-tool usage guidance to the MCP tool descriptions

Allow admins to add contextual rules about when and where to use a tool, specific to one instance, and the agent needs to see them before it picks a tool, not after.

Add a per-wiki, per-tool usage guidance text to the MCP configuration, editable in the MCP administration section next to the tool's enable toggle.

• When set, the guidance is appended to the tool's description in tools/list as a separate paragraph, in the same way the man hint is appended to the server description.
• The built-in description is never replaced. It carries the "how" (parameter semantics, limits, cross-wiki variants computed at list time, warnings) and is maintained with the tool; the guidance carries the instance's "when and where". Keeping the two apart also means the guidance survives upgrades of the built-in text.
• Clearing the field restores the stock description, which gives the "reset" behaviour
• Changing it invalidates the cached per-wiki server, as the other configuration fields already do, so the next connection sees the new list.