Turn docs/providers.md into an index and repoint its inbound links

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/plans-executions/2026-09-16-1108-provider-docs-split.md

@@ -6,7 +6,7 @@ Plan: [docs/plans/2026-09-16-1108-provider-docs-split.md](../plans/2026-09-16-11
 - [x] Step 1 — Write `docs/provider-contract.md`
 - [x] Step 2 — Write `docs/provider-spec-template.md`
 - [x] Step 3 — Write `docs/providers-existing.md`
-- [ ] Step 4 — Rewrite `docs/providers.md` as the index; link edits
+- [x] Step 4 — Rewrite `docs/providers.md` as the index; link edits
 - [ ] Step 5 — Verification sweep
 - [ ] Step 6 — Final summary, CHANGELOG after confirmation
 
@@ -121,3 +121,22 @@ Worth noting: matrix rows that the profile table now carries (Create
 non-blocking, GC marks, IP source, quota detection, Checker presence,
 credentials, logging, tracing, tests) were dropped from "Other differences"
 rather than kept twice.
+
+## Step 4 — Rewrite `docs/providers.md` as the index; link edits
+
+2026-09-16 11:40. `docs/providers.md` was rewritten in place (same path, so
+the inbound links from `README.md`, `CLAUDE.md`, `docs/architecture.md` and
+the historical plan files still resolve, and the pre-split blueprint stays
+reachable in the path's git history). The three inbound sentences were
+updated to name the new files:
+
+- `README.md` "### Adding a provider": the first sentence now names the
+  contract, the template and the appendix, with `docs/providers.md` as the
+  index; the "Start there" sentence is unchanged.
+- `CLAUDE.md` "## Architecture": the providers.md sentence now names all
+  three files and the index.
+- `docs/architecture.md` components row "Provider contract": "rules in
+  provider-contract.md, per-provider profile table in providers-existing.md".
+
+The historical files under `docs/plans/` and `docs/plans-executions/` keep
+their `providers.md` links on purpose.