docs: add opt-in survey recruitment banner to the docs site

[jayjdub]

Summary

Adds an opt-in, dismissible research recruitment banner to the docs site (docs/) that links
to a survey and is shown only to recruited participants. Adapted from the
github/research-accelerator recruitment-banner skill.

Survey: GitHub Agentic Workflows (Alchemer) · Slug: gh-aw-docs-research-2026q3

Contributing path

I'm not on the core team, so per CONTRIBUTING.md this is an issue rather than a PR (non-core PR
creation is blocked). The change is fully implemented on a fork branch — diff ready to review:
👉 https://github.com/github/gh-aw/compare/main...jayjdub:gh-aw:add-docs-recruitment-banner?expand=1
(branch jayjdub:gh-aw@add-docs-recruitment-banner). A core-team member can open the PR or
cherry-pick the branch.

Why this design (static-site targeting)

The stafftools recruitment banner targets current_user server-side. This docs site is static
GitHub Pages (github.github.com/gh-aw/) — no server, no current_user. Targeting is enforced
by who the recruitment link is distributed to (the dotcom_id audience):

• The link carries ?recruit=gh-aw-docs-research-2026q3 (optionally &uid=<dotcom_id>).
• Only recipients see the banner; eligibility is remembered per-browser (localStorage) and
  persists across docs pages; dismissal sticks.
• The audience list is never committed (committing participant IDs to a public repo would
  expose them).
• CTA gets utm_source=inproduct_banner&utm_medium=docs&utm_campaign=<slug> (+ pid); a
  banner_research_cta event fires on click.

Proposed changes (all under docs/)

File	Purpose
docs/src/config/recruitmentBanner.ts	Knobs: enabled, slug, copy, ctaUrl, path scope, frequency
docs/src/components/RecruitmentBanner.astro	Banner UI + client eligibility; overrides Starlight's Banner slot (preserves native frontmatter banners)
docs/astro.config.mjs	One line wiring the Banner override
docs/RECRUITMENT_BANNER.md	Config guide + go-live checklist

Safe by default

• Ships OFF (enabled: false) — landing it does not make the banner visible; turning it
  on is a deliberate follow-up change.
• No auto-cap — monitor and flip enabled: false at quota.
• No participant IDs committed; the gift-card incentive is stated honestly in the copy (payout
  is a separate flow).

Test plan

With enabled: true on a preview build, open any docs page with
?recruit=gh-aw-docs-research-2026q3&uid=<own-dotcom_id> → banner renders, CTA points at the
survey with UTM params, dismiss persists. Without ?recruit=, the banner never shows.

[jayjdub]

cc @cmuto09 — flagging you on this since it's tied to the GitHub Agentic Workflows survey recruitment.

The banner is fully implemented on a fork branch (diff ready for a core member to open as a PR or cherry-pick):
https://github.com/github/gh-aw/compare/main...jayjdub:gh-aw:add-docs-recruitment-banner?expand=1