docs: add board-operator delegation guide

Create docs/guides/board-operator/delegation.md explaining the full
CEO-led delegation lifecycle from the board operator's perspective.
Covers what the board needs to do, what the CEO automates, common
delegation patterns (flat, 3-level, hire-on-demand), and a
troubleshooting section that directly answers the #1 new-user
confusion point: "Do I have to tell the CEO to delegate?"

Also adds a Delegation section to core-concepts.md and wires the
new guide into docs.json navigation after Managing Tasks.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

docs/docs.json

@@ -46,6 +46,7 @@
               "guides/board-operator/managing-agents",
               "guides/board-operator/org-structure",
               "guides/board-operator/managing-tasks",
+              "guides/board-operator/delegation",
               "guides/board-operator/approvals",
               "guides/board-operator/costs-and-budgets",
               "guides/board-operator/activity-log",