[deep-report] quick-start.mdx: add inline YAML frontmatter snippet at first mention

### Description
`docs/src/content/docs/setup/quick-start.mdx` line 53 introduces YAML frontmatter with only a prose parenthetical ("the configuration block between the `---` markers at the top of the file") and no concrete example. A first-time reader has to infer the shape of frontmatter before seeing any workflow file. Verified still present in current source (no code block near the first mention).

### Expected Impact
A tiny inline snippet (e.g. a 3-line `---\non: schedule\n---` block) right at first mention removes ambiguity for brand-new users before they run their first command — a low-effort readability win flagged directly by the Documentation Noob Test Report.

### Suggested Agent
Docs-focused agent / Documentation Noob Tester follow-up.

### Estimated Effort
Fast (< 30 min)

### Data Source
DeepReport Intelligence Briefing analysis run, 2026-09-15, based on Documentation Noob Test Report discussion #61022. Verified live against `docs/src/content/docs/setup/quick-start.mdx:53` before filing — no existing open issue covers this specific gap.

> [!WARNING]
>
> Firewall blocked 1 domain
>
> The following domain was blocked by the firewall during workflow execution:
>
> - `api.anthropic.com`
>
> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "api.anthropic.com"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

> Generated by [🔬 Deep Report](https://github.com/github/gh-aw/actions/runs/34937799028) · claude · agent · 248 AIC · ⌖ 10.3 AIC · ⊞ 12.8K · [◷](https://github.com/search?q=repo%3Agithub%2Fgh-aw+is%3Aissue+%22gh-aw-workflow-call-id%3A+github%2Fgh-aw%2Fdeep-report%22&type=issues)
> - [x] expires on Sep 16, 2026, 10:52 PM UTC-08:00

Open docs/src/content/docs/setup/quick-start.mdx around line 53 and read the first explanation of YAML frontmatter. Add a short inline example at that first mention, then check the rendered quick-start page to confirm the snippet is clear and appears before the first workflow file example.

Written by the indexing model from the issue text.