task: #3613913 Call handing a task to another user reassignment, not delegation, so the word is free for delegating authority

Wording only, no behavior change.

The inbox's Reassign hands a task to another user and the docs call the assignee-initiated form of it "delegation". It is the same operation either way: ownership moves at once and for good, nothing is time-bounded, nothing comes back, and the task keeps no memory of who held it before. That is a transfer, so this calls it reassignment everywhere and frees "delegation" for the two features that need it, #3613914 and #3613915.

Changed: the Reassignment and delegation heading and paragraph in docs/human-tasks.md (plus a sentence spelling out that reassignment is a transfer, and pointing at delegation for the case it does not cover), the roadmap bullet, and the docblocks and comments in ReassignForm, InboxController::reassignAccess(), TaskActions and WorkItemManager::reassign(), and in TaskReassignTest.

No user-facing string changed, so no translation update is needed. The other uses of "delegate" in the codebase are the ordinary "hands off to" sense (TimeoutSweeper, IncidentManager, LocalOrchestraClient, ...) and are left alone.