Distinguish a real user-dismiss from a pipeline regeneration (status=deleted conflates them)

**Severity:** 🟠 inflates the "dismissed" count
**Group C — Postgres prompts**

## Problem
The `prompts` table has one `status=deleted` flag that mixes (a) a customer really rejecting a prompt with (b) the pipeline regenerating and superseding an old prompt with a near-duplicate. Any acceptance dashboard reading `status=deleted` as "dismissed" **over-counts rejections**.

## Evidence
Both the user-dismiss and the pipeline-regeneration go through the **same two functions**, with no reason recorded — which is exactly why they're indistinguishable:
- `softDeletePrompt` → `src/support/prompts-storage.js:1287` `.update({ status: 'deleted', updated_by })`
- `bulkSoftDeletePrompts` → `src/support/prompts-storage.js:1331` (same update)

Independent detection (from analysis): superseded deletions sit at cosine ≈1.00 to a still-kept prompt; for some brands the deleted rows are dominated by regeneration.

## Implementation
Add a `deleted_reason` (`user_dismiss` | `superseded`) — or a `superseded_by` / `superseded_at` pointer — param to those two functions, and have the **regeneration caller** pass `superseded` while the UI dismiss passes `user_dismiss`. Precedent already in this repo: `superseded_at` is used on another entity (`src/dto/audit-policy.js:76`).

## Retrieval impact
The true dismiss count **isn't retrievable** — `status=deleted` returns real rejects + pipeline regenerations mixed together, inflating "dismissed".

## Definition of done
A dismissed-because-rejected prompt is distinguishable from a regenerated one; acceptance metrics count only real dismisses.

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Related epic: adobe/project-elmo-ui#3108.

Start by tracing softDeletePrompt and bulkSoftDeletePrompts in src/support/prompts-storage.js at lines 1287 and 1331, then identify the regeneration caller and the UI dismiss path. Review the prompts status update and the precedent in src/dto/audit-policy.js:76; done means deleted prompts preserve whether they were user-dismissed or superseded and acceptance metrics count only real dismissals.

Written by the indexing model from the issue text.