feat(FN-5196): return 410 Gone for hard-archived task deletions

Implement the hard-archived delete contract: add a typed `ArchivedTaskDeleteError`, map hard-archived task deletes to HTTP 410 Gone at both the store and routes layers, and document the invariant in the soft-delete verification matrix.

Fusion-Task-Id: FN-5196

.changeset/fn-5196-archive-delete-contract.md

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+FN-5196: pin the archived-task delete contract. `deleteTask` on a hard-archived task now throws the typed `TaskAlreadyHardArchivedError`; the dashboard delete route maps this to HTTP 410 Gone with `code: "TASK_ALREADY_HARD_ARCHIVED"`. No silent no-op.