Add AGENTS.md for AI and contributor guidance (#1333)

Author: DanGould

AGENTS.md

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+# AGENTS.md
+
+For the full human guide see [`.github/CONTRIBUTING.md`](.github/CONTRIBUTING.md).
+
+Nightly Rust toolchain (`rust-toolchain.toml`) — required for `cargo fmt`
+unstable options.
+
+## Commit Rules
+
+Every commit must pass CI independently.
+
+Commit messages follow the seven rules:
+
+1. Separate subject from body with a blank line
+2. Limit the subject line to 50 characters
+3. Capitalize the subject line
+4. Do not end the subject line with a period
+5. Use the imperative mood in the subject line
+6. Wrap the body at 72 characters
+7. Use the body to explain what and why vs. how
+
+## Pre-commit Checks (Tiered)
+
+**Fast (every commit):**
+
+```sh
+cargo fmt --all -- --check
+cargo clippy --all-targets --keep-going --all-features -- -D warnings
+codespell
+```
+
+**Full (before push):**
+
+```sh
+./contrib/lint.sh
+RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --all-features \
+  --document-private-items
+./contrib/test_local.sh            # does NOT include payjoin-mailroom
+npx prettier --check '**/*.md'     # CI runs prettier via treefmt
+codespell
+```
+
+Per-crate scripts: `{crate}/contrib/test.sh`, `{crate}/contrib/lint.sh`.
+
+## Two Lockfiles
+
+CI tests both `Cargo-minimal.lock` and `Cargo-recent.lock`. After any
+dependency change:
+
+```sh
+bash contrib/update-lock-files.sh
+```
+
+## AI Disclosure
+
+Required in PR body. Do **not** add `Co-Authored-By` in commits.

CLAUDE.md

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+AGENTS.md