[product-manager-impersonator] Manual target-issue override for Scheduled Fix

## Feature Idea

**Summary:** Add an optional `target-issue-number` input to Scheduled Fix so manual runs can deterministically fix one chosen issue instead of scanning the queue.

## Why a Customer Would Want This
Teams running scheduled audits/fixes often need to fast-track or retry a specific known issue. Today, a manual `workflow_dispatch` run still follows prefix/label search, which is less predictable and can pick a different issue than the operator intended.

## Rough Implementation Sketch
- Add optional `target-issue-number` to `.github/workflows/gh-aw-scheduled-fix.md` `workflow_call` inputs and plumb it into the prompt instructions.
- Update candidate selection logic: if `target-issue-number` is provided, fetch that issue directly and skip prefix/label queue search.
- Keep existing `title-prefix` / `issue-label` behavior as the fallback for scheduled operation.
- Document the new input and a manual-run example in `gh-agent-workflows/scheduled-fix/README.md` and `example.yml`.

## Why It Won't Be That Hard
This is a small, additive input + prompt-branch change in one base workflow, and the repo already uses the same targeting pattern in detector/fixer chaining.

## Evidence
- Scheduled Fix currently has no direct issue selector in inputs: `.github/workflows/gh-aw-scheduled-fix.md` (lines 21-59).
- Current selection behavior is queue search by title prefix/label: `.github/workflows/gh-aw-scheduled-fix.md` (lines 118-130).
- Manual trigger exists, so deterministic manual targeting is a natural extension: `gh-agent-workflows/scheduled-fix/README.md` (lines 21-27).
- Existing precedent for targeted issue execution: `docs/workflows/detector-fixer-chaining.md` (lines 29-33, 93-97) uses `target-issue-number` with `gh-aw-create-pr-from-issue`.
- Duplicate checks returned no matching open request:
- `repo:elastic/ai-github-actions is:issue ("target-issue-number" AND "scheduled fix")` (0 results)
- `repo:elastic/ai-github-actions is:issue is:open in:title "[product-manager-impersonator]" "target-issue-number"` (0 results)
- `/tmp/previous-findings.json` has no matching title.

> [!NOTE]
>
> 🔒 Integrity filter blocked 16 items
>
> The following items were blocked because they don't meet the GitHub integrity level.
>
> - elastic/ai-github-actions#1067 `list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#359](https://github.com/elastic/ai-github-actions/issues/359) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#1067](https://github.com/elastic/ai-github-actions/issues/1067) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#704](https://github.com/elastic/ai-github-actions/issues/704) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#694](https://github.com/elastic/ai-github-actions/issues/694) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#684](https://github.com/elastic/ai-github-actions/issues/684) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#875](https://github.com/elastic/ai-github-actions/issues/875) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#676](https://github.com/elastic/ai-github-actions/issues/676) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#139](https://github.com/elastic/ai-github-actions/issues/139) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#1025](https://github.com/elastic/ai-github-actions/issues/1025) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#132](https://github.com/elastic/ai-github-actions/issues/132) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#235](https://github.com/elastic/ai-github-actions/issues/235) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#248](https://github.com/elastic/ai-github-actions/issues/248) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#191](https://github.com/elastic/ai-github-actions/issues/191) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#478](https://github.com/elastic/ai-github-actions/issues/478) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#593](https://github.com/elastic/ai-github-actions/issues/593) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
>
> To allow these resources, lower `min-integrity` in your GitHub frontmatter:
>
> ```yaml
> tools:
> github:
> min-integrity: approved # merged | approved | unapproved | none
> ```
>
>

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Product Manager Impersonator](https://github.com/elastic/ai-github-actions/actions/runs/25855840296)

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