docs(adr): add the ADR index

CONTRIBUTING.md

@@ -146,7 +146,8 @@ as an **Architecture Decision Record** (MADR format) under
 cargo xtask new-adr 'my new decision'
 ```
 
-Reference the ADR in the commit body. Look at
+Add a row to the index ([`docs/ADR_INDEX.md`](docs/ADR_INDEX.md)) and
+reference the ADR in the commit body. Look at
 `docs/adr/0008-MOVED.md` (and the canonical text in the sibling
 repo) for an example of a decision that fundamentally reshaped the
 pipeline; look at `docs/adr/0011-brand-boundary-css-class-rewrite.md`

docs/ADR_INDEX.md

@@ -0,0 +1,33 @@
+# Architecture Decision Records
+
+This directory holds [MADR 4.0](https://adr.github.io/madr/) Architecture
+Decision Records. Each file documents one decision; once accepted an ADR is
+never edited — it is *superseded* by a later ADR that links back.
+
+Several early ADRs **moved** to the sibling [`P4suta/aozora`](https://github.com/P4suta/aozora)
+repo when the parser core was extracted (ADR-0010); their numbers are kept as
+redirect stubs here so existing links don't rot.
+
+| ADR                                                          | Title                                                              | Status                        |
+| ------------------------------------------------------------ | ------------------------------------------------------------------ | ----------------------------- |
+| [0001](./adr/0001-fork-comrak-vendor-in-tree.md)             | Fork comrak and vendor it in-tree (0-line diff budget)             | accepted                      |
+| [0002](./adr/0002-docker-only-execution.md)                  | Every dev operation runs inside Docker                             | accepted                      |
+| [0003](./adr/0003-afm-parser-architecture.md)                | afm-parser architecture (trait-object extension, arena AST)        | superseded by 0010            |
+| [0004](./adr/0004-MOVED.md)                                  | → `aozora/docs/adr/0003-accent-decomposition-preparse.md`          | moved                         |
+| [0005](./adr/0005-paired-block-container-hook.md)            | Paired block annotation container hook                             | superseded by 0008            |
+| [0006](./adr/0006-MOVED.md)                                  | mirrored → `aozora/docs/adr/0004-lint-profile-policy.md`           | moved                         |
+| [0007](./adr/0007-MOVED.md)                                  | → `aozora/docs/adr/0005-corpus-sweep-strategy.md`                  | moved                         |
+| [0008](./adr/0008-MOVED.md)                                  | → `aozora/docs/adr/0001-zero-parser-hooks.md`                      | moved                         |
+| [0009](./adr/0009-authoring-tools-live-in-sibling-repositories.md) | Authoring tools live in sibling repositories                 | accepted                      |
+| [0010](./adr/0010-extract-aozora-core.md)                    | Extract aozora parser core into sibling repository `aozora`        | accepted                      |
+| [0011](./adr/0011-brand-boundary-css-class-rewrite.md)       | Brand boundary: HTML class rewrite at the afm side                 | accepted                      |
+
+## Authoring a new ADR
+
+1. Scaffold with `cargo xtask new-adr 'my new decision'` (copies
+   `adr/0000-template.md` to the next sequential number).
+2. Fill in the sections; keep paragraphs short and action-oriented.
+3. Add a row to the table above.
+4. Reference the ADR in the commit body and open a PR. ADRs are normally
+   accepted on merge; controversial ones land as `proposed` and flip to
+   `accepted` once the discussion concludes.