docs: sync canonical provider count 354 -> 356

[jonlwheat2-gif]

What was broken

release/v3.8.51 failed npm run check:docs-counts-sync with 5 STRICT drifts — the canonical provider count changed when Opper (#11629) and 1min.ai (#11631) boarded the catalog, but the count-derived docs were not synced. This was one of the Docs accuracy (fabricated-docs + i18n mirrors, strict) failures on PR #11644's CI.

Red evidence (before)

• Provider reference total (doc vs live modules): 356 (real) [STRICT]
  ✗ docs/reference/PROVIDER_REFERENCE.md — doc total 354 is stale — the live provider modules define 356
• SVG canonical numbers (live code): 356 providers ... [STRICT]
  ✗ docs/diagrams/readme-hero.svg — stale: "354 providers", "354 AI providers"
  ✗ docs/diagrams/promise-pillars.svg — stale: "354 providers"
  ✗ docs/diagrams/comparison-table.svg — stale: "354 providers"
  ✗ docs/diagrams/cli-terminal.svg — stale: "354 providers"
✗ 5 STRICT drift(s) detected.

After regenerating the reference, the downstream checks also required the root docs to mention 356:

✗ README.md does NOT mention "356" for providers
✗ AGENTS.md does NOT mention "356" for providers
✗ llm.txt does NOT mention "356" for providers
✗ package.json — description does not mention the live provider count 356

Fix

1. Regenerate the reference — npm run gen:provider-reference → docs/reference/PROVIDER_REFERENCE.md now reports Providers: 356 unique IDs.
2. Update the four README SVG diagrams (docs/diagrams/readme-hero.svg, promise-pillars.svg, comparison-table.svg, cli-terminal.svg): 354 providers / 354 AI providers → 356.
3. Update the root docs: README.md (hero + pillar + comparison alt-text, provider-catalog bullets, "356 AI Providers" heading), AGENTS.md line 49 (354 LLM providers → 356), llm.txt (3 claims), package.json description.
4. Re-sync the 42 docs/i18n/*/llm.txt mirrors via scripts/i18n/sync-llm-mirrors.mjs.

Green evidence (after)

node scripts/check/check-docs-counts-sync.mjs  →  ⚠ 2 potential (soft) drift(s) [non-blocking]
npm run check:docs-sync                        →  PASS - documentation version sync is consistent
npm run check:docs-all                         →  exit 0 (docs-sync PASS, doc-links PASS)
tests/unit/check-docs-counts-sync.test.ts      →  ℹ tests 28  ℹ pass 28  ℹ fail 0

Files changed (52)

Group	Files
Reference	docs/reference/PROVIDER_REFERENCE.md (regenerated)
Diagrams	docs/diagrams/{readme-hero,promise-pillars,comparison-table,cli-terminal}.svg
Root docs	README.md, AGENTS.md, llm.txt, package.json
Mirrors	42 × docs/i18n/*/llm.txt
Fragment	changelog.d/maintenance/11449-provider-count-docs-sync.md

Fixes the base-red docs drift tracked in #11449 that surfaced on PR #11644's CI.