[information-architecture] Product Manager Impersonator appears in conflicting workflow categories across docs

## Information Architecture Findings

### 1. Product Manager Impersonator is grouped inconsistently across the main workflow information architecture
**Area:** Consistency / Grouping and hierarchy
**Component(s):** `gh-agent-workflows/README.md`, `docs/workflows/gh-agent-workflows.md`, `mkdocs.yml`, `docs/index.md`

**Problem:**
The same workflow is categorized in different sections depending on entry point:
- In `gh-agent-workflows/README.md` it is listed under **Detectors** (`gh-agent-workflows/README.md:25-33`).
- In `docs/workflows/gh-agent-workflows.md` it is listed under **Standalone scheduled workflows** (`docs/workflows/gh-agent-workflows.md:85-98`).
- In docs navigation (`mkdocs.yml:59-66`) it is placed under **Planning & Insights**.
- The homepage frames it as an ideation workflow in **Idea Machines** (`docs/index.md:144-149`).

These competing groupings create conflicting mental models about what the workflow is for and where to find related workflows.

**Suggested improvement:**
Standardize Product Manager Impersonator under one taxonomy (for example, planning/insights) across all workflow catalogs and navigation surfaces, then align section headers and table groupings to that taxonomy.

## Suggested Actions
- [ ] Choose one canonical category for Product Manager Impersonator (e.g., Planning & Insights).
- [ ] Update `gh-agent-workflows/README.md` section membership to match the canonical category.
- [ ] Update `docs/workflows/gh-agent-workflows.md` grouping to match the same category.
- [ ] Verify homepage (`docs/index.md`) and docs nav (`mkdocs.yml`) use the same grouping language.

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