docs: provider-count single source of truth, RFC status sweep, adding-a-provider checklist

[eric8810]

Bundles the four documentation-consistency issues from the #166 docs track into one PR (per maintainer decision; each is still individually closable).

#182 — RFC status lines

Verified every status against the code before writing it:

• 0016 / 0020 / 0021 / 0022: DRAFT → IMPLEMENTED with landing PR refs. Beyond the issue's list, 0020 (external provider config) had also landed (feat(providers): external provider config overlay (RFC-0020) #97: register_provider runtime overlay + FFI/Node/Python passthrough) — the sweep missed it. 0016's §7.2 had four stale open items (M6 proxy / M7 aggregation / M11 stream aggregation / M12 generateObject, all landed via feat(core): M11 streamText aggregation + M12 generateObject + M6 proxy + rawFinishReason #99) — struck through with evidence pointers.
• 0005-rename: "pending execution" → EXECUTED (repo/crates/FFI all renamed; the RFC's own body was bulk-replaced, which is why some of its prose reads tautologically — noted in the status line).
• Missing status lines added: 0002 (IMPLEMENTED), 0003 (IMPLEMENTED, in force), 0004 (SUPERSEDED — snapshot, source of truth is the registry), 0005-protocol (ACCEPTED — the "no cross-protocol conversion" decision record), 0027-coverage (IMPLEMENTED, baseline snapshot).
• Bonus: the RFC-0031 links in 0009/0016 were broken (the RFC lives at docs/ai-sdk-request-pipeline.md, not rfc/) — fixed.

#177 — one source of truth for the provider count

The count was wrong in both directions: README said 329 in two places and 325 in two others; PROJECT-OVERVIEW/API/reference/CONTRIBUTING carried more stale copies.

• Fixed the counting source first: gen_providers_doc.py previously subtracted a hand-kept list of 2 machinery modules, so replay and catalogue were counted as providers. A module now counts iff it re-exports a typed surface (*Provider / *Config / *Model) — all four machinery modules drop out without a deny-list.
• True numbers: 251 registry + 76 typed = 327. The generator emits a Totals table into providers.md (registry rows, per-category typed providers, grand total; section titles truncated to first sentence for the table).
• Every living doc now references the page instead of repeating a number; README states the total once with a date and a link (tagline number now links to the page; the badge is the only other repetition, explicitly acknowledged). The two hand-maintained "coverage" tables (README / PROJECT-OVERVIEW) used groupings that can't be kept in sync with the generator's categories and were replaced with pointers + example lists. Historical snapshots under docs/internal/ and docs/quality-audit/ keep their numbers untouched.

#172 — gen_providers_doc.py --check in CI

• --check mode matching the gen_provider_names.py pattern (compare generated text, STALE + exit 1 on mismatch).
• CI step added in the contract-tests job right next to the existing ProviderName drift check.
• docs/api/providers.md regenerated once so the first run is green (verified locally: --check passes, gen_provider_names.py --check still green, ci.yml parses).

#173 — adding-a-provider checklist

New docs/contributing/adding-a-provider.md, written against the current repo mechanics (all commands and paths verified):

• Case 1 registry row: JSON row shape (incl. the real profile flags), both generators, cassette derivation via generate_thin_wrapper_cassettes.py (real OpenAI recordings, not fake data), roadmap note that Tracking: code reduction roadmap (providers, FFI, bindings, docs) #166 B2 replaces this with registry-iterating conformance.
• Case 2 new protocol: module layout, the section-comment → doc-category coupling in lib.rs, cassettes + conformance module, and the explicit warning that protocol providers are not name-addressable until Tracking: code reduction roadmap (providers, FFI, bindings, docs) #166 B1.
• Case 3 single modality: the Config/Provider-shell/one-modality-model pattern with serper.rs / lmnt.rs as references.
• The generator rule (do-not-edit header + CI --check), the provider-count rule, and the retirement conditions of every remaining scripts/ tool per Tracking: code reduction roadmap (providers, FFI, bindings, docs) #166.
• Linked from README (docs table) and CONTRIBUTING.md's existing "Adding a provider" section; CONTRIBUTING's stale "325" fixed too.

Verification

• python3 scripts/gen_providers_doc.py --check ✅ (and --check for gen_provider_names.py unchanged ✅)
• ci.yml valid YAML; new step mirrors the existing one
• All added relative links resolve (incl. the provider_registry.json path and the #totals anchors)
• No provider-count literals remain in any living doc (grep-verified; badge + one dated README statement excepted)