Implements the path-aware, two-tier documentation accessibility CI gate

[aheritier]

described in the shared plan docs-a11y-smart-gate (prior context:
docs-a11y-audit, shipped as PR #3687).

What changed

PR 1 — dark-mode callout-link contrast fix (docs/css/style.css)
Links inside a callout (warning/info/tip) sat on a variant-colored panel,
not the page background, so the shared prose-link color
(--accent-hover) failed there even though it passes everywhere else:
3.95:1 on the dark warning callout background (#643700), below the
4.5:1 AA threshold. This was a pre-existing bug, first surfaced while
pre-verifying features/tui/ as a new archetype (see below) — that page
has a link inside a [!WARNING] callout at line 503.

Scoped .content .callout a:not(.btn) { color: var(--blue-200); } (plus
a light-mode override restoring --accent, and an explicit :hover
rule so the shared --link-hover token isn't shadowed by the new
equal-specificity selector). Verified contrast ratios:

Background	--blue-200 (dark)	--accent (light, unchanged)
warning #643700	6.46:1	4.83:1
info #00153C	11.50:1	4.60:1
tip #11371A	8.51:1	4.75:1

The fix is intentionally scoped to callout links only — nothing broader
was touched.

PR 2 — scripts/docs-a11y-urls.sh (new)
Maps changed docs Markdown files to rendered URLs for Tier 2 of the
gate: docs/index.md → /, docs/404.md → /404.html,
docs/<section>/<page>/index.md → /<section>/<page>/ for the 8
mounted sections, _index.md/STYLE.md/non-Markdown files dropped
(silently or via ::notice for unexpected locations), de-dupes against
the static archetype list already in docs/.pa11yci.json, live-probes
survivors to catch renames, caps at ${A11Y_MAX_PAGES:-10} pages
(lexicographic, deterministic), and emits both ?theme=light and
?theme=dark for each surviving page. Ships with --self-test
(fixture-driven, no server required) and a header comment documenting
the mapping rules and env vars.

PR 3 — expand the static archetype list 8 → 12 URLs
(docs/.pa11yci.json)
Kept the existing 4 pages (home, configuration/sandbox/,
getting-started/quickstart/, 404.html) and added 2 more layout
archetypes: features/tui/ (the densest page — images, inline HTML,
many callouts/tables) and concepts/multi-agent/ (deep heading
structure). Each listed in both themes, so this tier is now 12 URLs.
Depends on PR 1 (the callout-link fix), since features/tui/?theme=dark
would otherwise fail immediately on the pre-existing bug.

PR 4 — wire Tier 2 into .github/workflows/docs-a11y.yml

• actions/checkout gets fetch-depth: 2 so git diff HEAD^1 HEAD
  (against the pull_request test-merge commit) yields exactly the
  PR's changed files — token-free, fork-safe, no changed-files action.
• A Compute changed-page URLs step (PR-only) runs the new helper
  script against that diff.
• An Assemble pa11y config step jq-merges the changed-page URLs into
  a generated config on top of the checked-in docs/.pa11yci.json
  (which stays the single source of truth for the static list and
  defaults, so the local contributor flow is unchanged).
• The Run pa11y-ci step uses the generated config when one exists,
  else falls back to the static-only config — so push-to-main and
  content-free PRs stay static-only (12 URLs), matching the plan (no
  PR diff to compute Tier 2 from on push).
• Workflow header comment rewritten to describe the two-tier model;
  scripts/docs-a11y-urls.sh added to both paths: trigger lists.

docs/STYLE.md updated in the same PRs that introduce each piece: the
callout-link contrast rule + table row (PR 1), and the two-tier
model/cap/archetype-maintenance guidance under "Running the
accessibility scan locally" (PR 4).

Deviations from the plan

None. All mechanisms (checked-in static config + generated merge file,
git-native diff, deterministic lexicographic cap at N=10, static+changed
for mixed PRs, 6-page/12-URL archetype list, callout-scoped contrast
fix) match the plan's recommendations as confirmed by the user. No
scope expansion beyond the callout-link fix.

Validation

• ./scripts/docs-a11y-urls.sh --self-test — all checks pass (mapping
  rules, de-dup, both-theme emission, cap + lexicographic tie-break +
  notice).
• shellcheck scripts/docs-a11y-urls.sh — clean.
• actionlint .github/workflows/docs-a11y.yml and
  ./scripts/workflow-lint.sh — clean.
• python3 -c "import json; json.load(open('docs/.pa11yci.json'))" —
  valid JSON.
• npx markdownlint-cli2 STYLE.md — 0 errors.
• Local hugo server (Hugo v0.164.0) + pa11y-ci@3.1.0 (system Chrome
  via PUPPETEER_EXECUTABLE_PATH, Docker was unavailable in this
  sandbox) against the full 12-URL static set, both themes: 12/12
  passed, 0 errors — including features/tui/?theme=dark, confirming
  the PR 1 fix.
• End-to-end simulation of the exact CI mechanism (helper script → jq
  merge → generated config → pa11y-ci) against a real changed-page
  example: 14/14 passed (12 static + 2 changed-page URLs, with a
  static-list page correctly de-duped away and a nonexistent/renamed
  page correctly dropped by the live-probe safety net).
• No dark-mode regression: only the callout-link token changed, and it
  doesn't touch anything on the docs-a11y-audit "verified passing —
  do not touch" list.

Final green CI is to be confirmed on GitHub once this PR is open — not
verifiable from the local sandbox for the GitHub Actions runner itself
(the local run stands in for the mechanism, using system Chrome instead
of browser-actions/setup-chrome).

Note: unrelated pre-existing uncommitted changes in the sandbox

The working tree also had uncommitted changes to docs/hugo.yaml,
docs/layouts/_partials/footer.html, and a new
docs/layouts/home.llms.txt (an in-progress llms.txt output-format
feature) that predate this task and are unrelated to the a11y gate.
They were deliberately left uncommitted/untouched — not included in any
commit here — so this PR stays scoped to the accessibility gate only.

Refs: plan docs-a11y-smart-gate, plan docs-a11y-audit, PR #3687.