[product-manager-impersonator] Manual-first quick setup mode

## Feature Idea

**Summary:** Add an optional `--manual-first` flag to `scripts/quick-setup.sh` that installs workflows with `schedule` disabled so teams can validate via `workflow_dispatch` before enabling automation.

## Why a Customer Would Want This
Teams trying GitHub Agent Workflows for the first time often want a safe rollout path: install everything, run a few manual tests, and only then turn on weekday schedules. Today quick setup installs scheduled triggers as-is, so merging the setup PR can immediately start automated issue/PR traffic.

## Rough Implementation Sketch
- Add `--manual-first` to `scripts/quick-setup.sh` CLI parsing and help text.
- After downloading each `trigger-*.yml`, strip `on.schedule` while preserving `workflow_dispatch` and other triggers.
- Print a post-install note listing which files were modified and how to re-enable schedules.
- Document the flag in quick-setup docs and workflow setup guidance.

## Why It Won't Be That Hard
This is a focused script-and-doc change: quick setup already centralizes trigger file download and already supports behavior toggles (`--dry-run`, `--continuous-improvement`). The workflow examples already include `workflow_dispatch`, so manual validation paths already exist.

## Evidence
- `scripts/quick-setup.sh:194-203` downloads trigger files verbatim, with no trigger-shaping step.
- `scripts/quick-setup.sh:268-270` immediately commits/pushes/opens a PR, so merge can activate schedules right away.
- `gh-agent-workflows/bug-hunter/example.yml:3-5` shows the common pattern of both `schedule` and `workflow_dispatch`.
- `gh-agent-workflows/product-manager-impersonator/example.yml:3-5` shows the same schedule+manual-trigger pairing.
- `docs/workflows/gh-agent-workflows.md:67-102` documents the broad set of scheduled workflows users may install.
- Distinct from `#877` (schedule staggering): this is staged rollout via temporary schedule suppression, not cron timing adjustment.

> [!NOTE]
>
> 🔒 Integrity filter blocked 21 items
>
> The following items were blocked because they don't meet the GitHub integrity level.
>
> - [#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".
> - [#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".
> - [#851](https://github.com/elastic/ai-github-actions/pull/851) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#589](https://github.com/elastic/ai-github-actions/pull/589) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#687](https://github.com/elastic/ai-github-actions/pull/687) `search_pull_requests`: 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".
> - [#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".
> - [#245](https://github.com/elastic/ai-github-actions/issues/245) `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".
> - [#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".
> - [#224](https://github.com/elastic/ai-github-actions/issues/224) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#767](https://github.com/elastic/ai-github-actions/pull/767) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#312](https://github.com/elastic/ai-github-actions/pull/312) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#287](https://github.com/elastic/ai-github-actions/issues/287) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#333](https://github.com/elastic/ai-github-actions/issues/333) `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".
> - ... and 5 more items
>
> 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/26158248375)

Give us feedback! React with 🚀 if perfect, 👍 if helpful, 👎 if not.