docs: add AGENTS.md + CLAUDE.md agent-contributor guidance

AGENTS.md

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+# AGENTS.md — coven-devkit
+
+Guidance for **AI agents** (Codex, Claude Code, and any Coven familiar) opening
+pull requests against this repo. Humans: your canonical guide is
+[`CONTRIBUTING.md`](CONTRIBUTING.md) — this is the agent-specific layer on top.
+
+> **Read first:** [`README.md`](README.md) for what this repo is, and
+> [`CONTRIBUTING.md`](CONTRIBUTING.md) for the full contribution bar — including
+> the **DCO sign-off requirement**, which is mandatory here.
+
+---
+
+## What this repo is (one line)
+
+`@opencoven/coven-devkit` is an **OpenClaw plugin package** that bundles narrow,
+repo-aware **skills** for expert Coven work.
+
+## DCO — every commit must be signed off (mandatory)
+
+This repo uses the **Developer Certificate of Origin**. Every commit needs a
+`Signed-off-by` trailer or merge will block it:
+
+```sh
+git commit -s -m "type: summary"
+```
+
+Use a real GitHub-linked identity. See [`CONTRIBUTING.md`](CONTRIBUTING.md) for
+the full DCO text.
+
+## Branch & PR workflow (all agents)
+
+- **Never push to `main`.** Every change lands via a PR with green CI. Branch
+  from current `origin/main`.
+- **Fresh branch per task**; use a worktree if multiple sessions may touch this
+  repo:
+  ```sh
+  git fetch origin main
+  git worktree add -b <branch> /tmp/devkit-<branch> origin/main
+  ```
+- Keep the diff scoped to one concern; conventional-commit subjects (`feat:`,
+  `fix:`, `docs:`, `chore:`, `refactor:`).
+- After merge: delete the remote branch, remove your local worktree/branch.
+
+## Checks — run locally before opening the PR
+
+```sh
+pnpm run check      # scripts/validate-skills.sh — every skill must validate
+```
+
+A skill that fails validation blocks the PR. Fix the skill manifest/structure
+rather than skipping the check.
+
+## Repo-specific invariants (don't break these)
+
+- Skills must stay **narrow and repo-aware** — one clear job each, not
+  general-purpose grab bags.
+- Every skill must pass `validate-skills.sh`. When you add or edit a skill,
+  keep its `SKILL.md` and any referenced assets consistent and self-contained.
+- This is a **plugin package** consumed by OpenClaw; keep the packaging surface
+  (`@opencoven/coven-devkit`) stable and intentional.
+
+## Attribution — credit contributors correctly
+
+When you re-land or build on someone else's work (a fork PR, an issue author's
+proposal, a co-author), **credit the human contributor with a working
+GitHub-linked trailer** so they appear in the contributors graph:
+
+```
+Co-authored-by: Full Name <ID+username@users.noreply.github.com>
+```
+
+- Use the **numeric-id no-reply form**. Get the id with `gh api users/<login> --jq .id`.
+- **Never** use a machine or `.local` email in a co-author trailer — it links to
+  no account and gives **zero** credit.
+- A commit can carry **both** `Signed-off-by:` (DCO, required) and
+  `Co-authored-by:` (attribution) trailers — include both when re-landing a
+  contributor's work.
+- Credit **people**, not AI tools.
+
+## Secrets & safety
+
+- Never commit secrets, tokens, or private emails. Use `*.noreply.github.com`
+  for attribution.
+- Don't disable CI gates or branch protection to land a change; surface the
+  blocker instead.
+
+## Claude Code
+
+`CLAUDE.md` points here — this file is the source of truth for both.

CLAUDE.md

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+# CLAUDE.md — coven-devkit
+
+**Read [`AGENTS.md`](AGENTS.md).** It is the canonical guide for AI agents
+(including Claude Code) contributing to this repo — the mandatory **DCO
+sign-off** (`git commit -s`), the branch/PR workflow, local checks
+(`pnpm run check` = `validate-skills.sh`), the skill invariants, and contributor
+attribution.
+
+Claude Code auto-loads this file; everything you need lives in `AGENTS.md` plus
+[`README.md`](README.md) and [`CONTRIBUTING.md`](CONTRIBUTING.md). There is no
+separate Claude-only workflow — follow `AGENTS.md` (and remember `-s` on every
+commit).