The dashboard's POST /links is the fourth writer of link_tasks (CLI, tool,
dashboard, plus the graph builder); return the same ``gated`` flag so every
surface that can create the deadlock can see it. Document the
``dependency_wait`` payload the link path emits and the delegation rule the
reporter derived (never link a support card under the card it unblocks).
Drops the CLI output test (a change-detector on prose); the two DB-level
invariants (event emitted on demotion / none for a done parent) stay.
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