docs: AGENTS.md — the role router, first file of the vendored set

'You are a reviewer here' becomes a sufficient launch prompt: agent
harnesses auto-load root AGENTS.md (the cross-agent convention); in a
governed repo a thin root stub forwards to the vendored
.ceremony/AGENTS.md, which routes each role to its file and states the
two rules every role shares (only triage mints issues; only humans
merge).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

AGENTS.md

@@ -0,0 +1,49 @@
+# AGENTS.md — start here
+
+You are an agent working in a repo governed by
+[heavy-duty/ceremony](https://github.com/heavy-duty/ceremony). This file is
+the router: find your role below, read its file, then act. The role files
+sit beside this one — in ceremony itself at the repo root, in a governed
+repo under `.ceremony/` (a machine-managed mirror; never edit those files
+in place — they are changed in heavy-duty/ceremony, through its own flow).
+
+## Your role
+
+You were told your role when you were pointed at this repo ("you are a
+reviewer here"). That one word is your whole onboarding:
+
+| you are the… | read | your job in one line |
+|---|---|---|
+| **triage** agent | [TRIAGE.md](TRIAGE.md) | turn discussions into buildable issues — or refuse well; you are the only door issues come through |
+| **builder** agent | [BUILDER.md](BUILDER.md) | turn one `ready` issue into one PR that meets its acceptance criteria |
+| **reviewer** agent | [REVIEWER.md](REVIEWER.md) | verdicts on PRs — approve or request-changes, converge, hand to the human |
+
+Everyone, whatever the role, also reads [LABELS.md](LABELS.md) — the labels
+are the shared state machine, and misusing one lies to every other agent on
+the board.
+
+**Not told a role?** Infer it from the task: asked to review a PR → reviewer;
+asked to implement an issue → builder; asked to process discussions or the
+backlog → triage. Still ambiguous → ask before acting. Do not free-lance
+across roles in one session: a builder reviewing its own PR, or a reviewer
+pushing fixes, breaks the separation the pipeline depends on.
+
+## The pipeline you are part of
+
+```
+discussion ──▶ triage ──▶ issue ──▶ build ──▶ review ──▶ human merge ──▶ release
+ (anyone)     (agent)   (queue)    (agent)   (agents)     (human)      (ceremony)
+```
+
+Two rules bind every role:
+
+- **Only triage mints issues.** Found work? Open or extend a discussion.
+- **Only humans merge.** Convergence ends at `state:needs-human`, never at
+  a merge button.
+
+## Repo specifics
+
+What is true only of *this* repo — the review panel roster, the `scope:*`
+label set, what a drill means, code conventions — lives in the repo's own
+`CONTRIBUTING.md`. Read it after your role file; where it and the role file
+disagree on a repo-specific fact, the repo's CONTRIBUTING wins.

CONTRIBUTING.md

@@ -88,11 +88,11 @@ Two consumption modes, split by what has a runtime:
 - **Doctrine is consumed as a machine-verified mirror.** A document's only
   "runtime" is an agent reading the working tree of the repo it stands in —
   a doc that requires a cross-repo fetch before it governs is a doc that
-  sometimes goes unread. So the agent-facing set — **TRIAGE.md, BUILDER.md,
+  sometimes goes unread. So the agent-facing set — **AGENTS.md, TRIAGE.md,
-  REVIEWER.md, LABELS.md** — is vendored into each governed repo at
+  BUILDER.md, REVIEWER.md, LABELS.md** — is vendored into each governed
-  **`.ceremony/`**, byte-identical to this repo at the pinned ref, by the
+  repo at **`.ceremony/`**, byte-identical to this repo at the pinned ref,
-  sync tool (issue #19). A CI guard diffs the mirror against the pin on
+  by the sync tool (issue #19). A CI guard diffs the mirror against the pin
-  every PR: hand-editing a vendored file, or bumping the pin without
+  on every PR: hand-editing a vendored file, or bumping the pin without
   re-syncing, goes red. It is a copy that cannot drift — which is the only
   kind of copy this org allows.
 
@@ -100,6 +100,12 @@ A governed repo (box, rig, cast, incubator, …) therefore carries:
 
 - `.ceremony/` — the vendored doctrine (machine-written; never edited by
   hand; agents read it from the checkout, no network, no other repo);
+- a thin root **`AGENTS.md` stub** — a few lines: "governed by
+  heavy-duty/ceremony; read `.ceremony/AGENTS.md` first; repo specifics in
+  CONTRIBUTING". The stub is what makes "you are a reviewer here" a
+  sufficient launch prompt: agent harnesses auto-load root AGENTS.md (the
+  cross-agent convention), and the vendored router takes it from there.
+  Tool-specific files (`CLAUDE.md`, …) reduce to one pointer line at it;
 - the thin workflow callers (release, labels) pinned to a ceremony tag, plus
   the `docs-sync --check` guard step in CI;
 - a short header in its own CONTRIBUTING pointing agents at `.ceremony/`,