[product-manager-impersonator] Manual persona overrides in trigger workflow

## Feature Idea

**Summary:** Add `workflow_dispatch` inputs to the Product Manager Impersonator trigger template so operators can override `persona`, `idea-size`, and `title-prefix` at run time without editing YAML.

## Why a Customer Would Want This
Teams running this workflow often want ad-hoc idea generation from different perspectives (for example, SRE vs. security) or a one-off `medium` idea run. Today, the shipped trigger template supports manual dispatch but exposes no form inputs, so users must change the workflow file and open a PR just to tune one run.

## Rough Implementation Sketch
- Add `workflow_dispatch.inputs` for `persona`, `idea-size`, and `title-prefix` in `gh-agent-workflows/product-manager-impersonator/example.yml`.
- Pass those values into `jobs.run.with` so manual runs can override defaults while scheduled runs remain unchanged.
- Keep defaults aligned with the reusable workflow contract (`""`, `"small"`, and `"[product-manager-impersonator]"`).
- Update `gh-agent-workflows/product-manager-impersonator/README.md` with a manual-run example using dispatch-time overrides.

## Why It Won't Be That Hard
The reusable workflow already supports these inputs; this is mostly wiring and docs. No runtime engine changes are needed, and the scope is a single trigger template plus its README.

## Evidence
- `gh-agent-workflows/product-manager-impersonator/example.yml#L2-L16` defines `workflow_dispatch` but no dispatch inputs and no `with:` passthrough.
- `gh-agent-workflows/product-manager-impersonator/README.md#L30-L37` documents `persona`, `idea-size`, and `title-prefix` as supported inputs.
- `.github/workflows/gh-aw-product-manager-impersonator.lock.yml#L72-L112` already declares those inputs in the reusable workflow.
- Open-issue duplicate checks found no matching request:
- `repo:elastic/ai-github-actions is:issue is:open "product-manager-impersonator" "workflow_dispatch" persona` → 0 results
- `repo:elastic/ai-github-actions is:issue is:open "persona" "idea-size" "product-manager-impersonator"` → 0 results
- `repo:elastic/ai-github-actions is:issue in:title "[product-manager-impersonator]" "persona"` → 0 results
- `/tmp/previous-findings.json` contains prior PM ideas (run names, quick-setup flags, scaffold generator, etc.) but no dispatch-time persona override proposal.

> [!NOTE]
>
> 🔒 Integrity filter blocked 33 items
>
> The following items were blocked because they don't meet the GitHub integrity level.
>
> - [#1175](https://github.com/elastic/ai-github-actions/pull/1175) `list_pull_requests`: 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".
> - [#1175](https://github.com/elastic/ai-github-actions/pull/1175) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#1128](https://github.com/elastic/ai-github-actions/pull/1128) `search_pull_requests`: 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".
> - [#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".
> - [#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".
> - [#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".
> - [#336](https://github.com/elastic/ai-github-actions/issues/336) `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".
> - [#354](https://github.com/elastic/ai-github-actions/issues/354) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#321](https://github.com/elastic/ai-github-actions/issues/321) `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".
> - elastic/ai-github-actions#1067 `list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - ... and 17 more items
>
> To allow these resources, lower `min-integrity` in your GitHub frontmatter:
>
> ```yaml
> tools:
> github:
> min-integrity: approved # merged | approved | unapproved | none
> ```
>
>

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[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Product Manager Impersonator](https://github.com/elastic/ai-github-actions/actions/runs/26753420316)

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