[product-manager-impersonator] One-click maintenance validate trigger setup

## Feature Idea

**Summary:** Add an optional quick-setup path that installs a scheduled `agentics-maintenance` **validate** trigger, so maintainers get recurring workflow validation without manual dispatch.

## Why a Customer Would Want This
Teams using this repo’s workflows can install many triggers quickly, but ongoing validation of workflow health still depends on someone remembering to manually run maintenance operations. A built-in “enable validation schedule” option gives maintainers early drift detection with less operational overhead.

## Rough Implementation Sketch
- Add a small installable trigger example (for example, `gh-agent-workflows/agentics-maintenance-validate/example.yml`) that calls `.github/workflows/agentics-maintenance.yml` with `operation: validate` on a conservative schedule plus `workflow_dispatch`.
- Add a `scripts/quick-setup.sh` flag (for example, `--maintenance-validate`) to install that trigger alongside existing workflow triggers.
- Update docs to show this as an optional day-2 ops add-on in the maintenance section.
- Keep default behavior unchanged unless the new flag is provided.

## Why It Won't Be That Hard
The core validation capability already exists and is production-wired; this is mostly trigger plumbing and docs. `agentics-maintenance` already supports `validate` and already has a `validate_workflows` job that files issues on findings, so implementation is limited to exposing that path ergonomically.

## Evidence
- Existing validate operation and validation job already implemented in maintenance workflow:
- `.github/workflows/agentics-maintenance.yml:54` (operation option includes `validate`)
- `.github/workflows/agentics-maintenance.yml:291` (`validate_workflows` job)
- `.github/workflows/agentics-maintenance.yml:323` (`Validate workflows and file issue on findings`)
- Quick setup currently installs `agentics-maintenance.yml` itself but no scheduled validate trigger:
- `scripts/quick-setup.sh:206-213`
- Docs currently describe manual maintenance operations (`safe_outputs`, `create_labels`) but not an installable recurring validate trigger path:
- `docs/workflows/gh-agent-workflows.md:157-162`
- Recent activity shows ongoing drift/operability pain where earlier detection helps:
- `#1225` (workflow drift detected)
- `#1231` (recurring agent run failures)

Duplicate check: compared against open PM and prior findings (including `#709`, `#769`, `#893`, `#965`, `#1212`, `#1220`) and did not find an issue proposing quick-setup installation of a scheduled `operation: validate` trigger.

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