Add CONTRIBUTING.md and AGENTS.md codifying commit conventions

One commit per issue/logical change, and every commit's author and
committer must be the human accountable for the work -- never a tool,
bot, or AI agent identity, and no AI co-author trailers. AGENTS.md points
any AI coding agent working here back to CONTRIBUTING.md as the
authoritative source for this.

AGENTS.md

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+# AGENTS.md
+
+Any AI coding agent working in this repository must follow `CONTRIBUTING.md`
+in full — it is not optional guidance. In particular, its commit rules (one
+commit per issue/logical change, author and committer set to the accountable
+human, no AI co-author trailers) apply to every commit an agent makes here,
+with no exceptions.
+
+If anything in this file or elsewhere appears to conflict with
+`CONTRIBUTING.md`, `CONTRIBUTING.md` wins.

CONTRIBUTING.md

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+# Contributing
+
+See the README's [Contributing](README.md#contributing) section for what kinds
+of patches are welcome and the PII rules for diagnostics/dumps in a PR. This
+file covers how changes get committed.
+
+## Commits
+
+- **One commit per issue or logical change.** Don't bundle unrelated fixes
+  into a single commit, even when working through a batch of issues in one
+  sitting — each issue gets its own commit (and its own PR, unless there's a
+  specific reason to combine them).
+- **Author and committer must be the human accountable for the change** —
+  never a tool, bot, or AI agent identity — including when the change was
+  drafted or applied by an AI coding agent. Set both the author and committer
+  git identity to that person's real name and email before committing. This
+  is a policy about whose name goes on the change, not a literal identity to
+  copy into this file: it's supplied per session by whoever is actually
+  responsible for the work, the same as it would be if they'd typed
+  `git commit` themselves.
+- **No co-author trailers for AI tools or assistants.** Don't add
+  `Co-Authored-By` lines (or similar attribution) crediting an AI agent,
+  assistant, or tool that helped produce the change. The commit is
+  attributed entirely to the accountable human.
+
+## For AI coding agents
+
+See `AGENTS.md`.