FN-5731: require confirmation before archiving near-duplicate tasks

Stop silent near-duplicate auto-archive by routing duplicates through explicit user confirmation in API, engine, and dashboard flows.

- add new near-duplicate confirmation metadata/types and preserve duplicate task rows until confirmation
- update workflow routes and triage logic to return confirmation-required outcomes instead of immediate archive
- add TaskCard and TaskDetailModal UI/actions plus styling and hook updates for confirming or rejecting duplicate handling
- expand dashboard and engine tests and task-management docs to cover the new confirmation path

Files changed:
docs/task-management.md                            | 14 ++++-
packages/core/src/types.ts                         |  4 ++
packages/dashboard/app/api/legacy.ts               |  1 +
packages/dashboard/app/components/TaskCard.css     | 59 ++++++++++++++++++
packages/dashboard/app/components/TaskCard.tsx     | 46 +++++++++++++-
packages/dashboard/app/components/TaskDetailModal.css   | 34 +++++++++++
packages/dashboard/app/components/TaskDetailModal.tsx   | 71 +++++++++++++++++++++-
packages/dashboard/app/components/__tests__/TaskCard.test.tsx     | 71 ++++++++++++++++++++++
packages/dashboard/app/components/__tests__/TaskDetailModal.rendering.test.tsx   | 68 +++++++++++++++++++++
packages/dashboard/app/hooks/useTasks.ts           |  2 +-
packages/dashboard/src/__tests__/routes-tasks-near-duplicate.test.ts  | 45 ++++++++++++++
packages/dashboard/src/routes/register-task-workflow-routes.ts    |  9 ++-
packages/engine/src/__tests__/reliability-interactions/near-duplicate-intake.test.ts                  | 16 +++--
packages/engine/src/triage.ts                      | 19 +++---
14 files changed, 433 insertions(+), 26 deletions(-)

Fusion-Task-Id: FN-5731

Fusion-Task-Lineage: fa004129-7bce-4457-b8c3-ceb9fb953319

packages/core/src/types.ts

@@ -1024,6 +1024,7 @@ export type ActivityEventType =
   | "task:duplicate-warning-overridden"
   | "task:auto-archived-deterministic-duplicate"
   | "task:auto-archived-near-duplicate"
+  | "task:near-duplicate-flagged"
   | "task:auto-archived-ghost-bug"
   | "task:auto-archived-duplicate"
   | "task:merge-worktree-reacquired"
@@ -1660,6 +1661,9 @@ export interface TaskSource {
    * Reserved metadata keys:
    * - `duplicateOfTaskIds: string[]` stores structured duplicate lineage captured
    *   from triage parsing and backfills.
+   * - near-duplicate markers: `nearDuplicateOf` (canonical task id),
+   *   `nearDuplicateScore` (number), `nearDuplicateSharedTokens` (string[]),
+   *   and optional `nearDuplicateDismissed` (boolean).
    */
   sourceMetadata?: Record<string, unknown>;
 }