feat(tasks): proposals suggest an assignee; assigning on the board accepts (v0.457.0)

[vikasprogrammer]

• task_create gains suggest — required whenever a task lands proposed; stored as tasks.suggested_assignee (advice, not an assignment). An assignee on a proposal is read as the suggestion; the agent's later task_update({assignee}) only revises it.
• Accepting a proposal with no assignee adopts the suggestion (Inbox Accept, Accept & run, board status → todo); dismiss doesn't.
• Board: assigning someone to a proposal moves it to todo (same gate as accept; audited task.proposal.accepted, via: 'assign'). No duplicate "assigned to you" notice.
• UI: suggestion shown as "suggested" on the Inbox card, the board's proposals strip, and the task page (with Assign & accept).

Tested: npm run test:governance green; scripts/task-proposals-test.cjs §9 pins the new behaviour (77 assertions).

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