2.8.5 MCP delete_work_item tool (permanent, subtree cascade)

Estimate: 30m · Type: code · Depends on: 2.8.2

MCP delete_work_item (key) tool → wraps the 2.8.2 deleteWorkItem service (permanent, subtree cascade). Permission-gated as the route; dual-content result (structuredContent: the deletion summary incl. deleted ids/count; text summary). Update docs/mcp.md (tool count + catalog) and note the archive-vs-delete distinction (archive_work_item reversible vs delete_work_item permanent). Depends on 7.8.4/7.8.5 (done).

Acceptance criteria

• delete_work_item permanently deletes the item + subtree via the service; returns the deleted set; permission denial → typed error; covered by the 7.8.9 round-trip suite.
• docs/mcp.md updated.

Context refs

• /api/mcp route + write-tool registration; 7.8.14 (archive/unarchive) sibling; docs/mcp.md