AGENTS.md: First version

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+# AGENTS
+
+This document describes the minimum environment required on build / CI agents to compile `dovecot-core` and run the unit tests. It assumes a POSIX-like system with a reasonably recent toolchain.
+
+---
+
+## 1. Required software on agents
+
+### 1.1 Toolchain and build tools
+
+Agents must have:
+
+- C compiler (GCC or Clang)
+- `make`
+- Autotools:
+  - `autoconf`
+  - `automake`
+  - `libtool`
+- `pkg-config`
+- `flex`
+- `bison`
+- `git`
+- `wget`
+- Scripting tools:
+  - `perl`
+  - `python3`
+
+When building from **git**, `autoreconf -vif` is used to bootstrap the autotools build system.
+
+### 1.2 Core libraries
+
+At minimum, agents should provide development headers and libraries for:
+
+- OpenSSL
+- `zlib`
+- PCRE2 (32-bit wide)
+
+### 1.3 Recommended libraries for broader test coverage
+
+Many unit tests exercise optional functionality; for full coverage, agents should have dev packages for:
+
+- SQL backends:
+  - SQLite
+  - PostgreSQL
+  - MySQL/MariaDB
+- Authentication / identity:
+  - PAM
+  - LDAP
+  - SASL (for libldap)
+- System integration:
+  - systemd (libsystemd)
+  - libcap
+- Compression / storage:
+  - bzip2
+  - LZ4
+  - LZMA / xz
+- Misc:
+  - ICU
+  - libsodium
+  - libunwind
+
+### 1.4 Example package sets
+
+#### Debian / Ubuntu
+
+```sh
+apt-get update
+apt-get install -y \
+  build-essential \
+  autoconf automake libtool pkg-config \
+  flex bison \
+  git wget \
+  perl python3 \
+  libssl-dev zlib1g-dev \
+  libpcre2-32-0 \
+  libpcre2-dev \
+  libsqlite3-dev libmysqlclient-dev libpq-dev \
+  libldap2-dev libpam0g-dev libsasl2-dev \
+  libsystemd-dev libcap-dev \
+  libicu-dev libcurl4-openssl-dev libunwind-dev \
+  liblz4-dev liblzma-dev libsodium-dev
+```
+
+#### RHEL / CentOS / Fedora
+
+```sh
+dnf install -y \
+  gcc gcc-c++ make \
+  autoconf automake libtool pkgconfig \
+  flex bison \
+  git wget \
+  perl python3 \
+  openssl-devel zlib-devel \
+  pcre2-utf32 \
+  pcre2-devel \
+  sqlite-devel mariadb-connector-c-devel postgresql-devel \
+  openldap-devel pam-devel cyrus-sasl-devel \
+  systemd-devel libcap-devel \
+  libicu-devel libcurl-devel libunwind-devel \
+  lz4-devel xz-devel libsodium-devel
+```
+
+---
+
+## 2. Building dovecot-core
+
+Standard sequence:
+
+```sh
+./configure
+make
+sudo make install
+```
+
+When building from **git**, run:
+
+```sh
+autoreconf -vif
+./configure
+make -j$(nproc)
+```
+
+To enable common features, use
+
+```sh
+autoreconf -vif
+./configure --with-pgsql --with-ldap --with-notify=inotify --with-lzma=auto --with-zlib --with-bzlib --with-lz4=auto --with-sql=plugin --with-sqlite --enable-maintainer-mode --with-cassandra --with-mysql --with-ldap=plugin --with-lua=plugin --with-solr
+make -j$(nproc)
+```
+
+NOTE: --enable-maintainer-mode allows rebuilding automake changes after Makefile.am changes in various directories.
+
+To see various available options, use
+
+```sh
+./configure --help
+```
+
+For non‑standard dependency paths:
+
+```sh
+CPPFLAGS="-I/opt/openssl/include" \
+LDFLAGS="-L/opt/openssl/lib" \
+./configure
+```
+
+---
+
+## 3. Running unit tests
+
+### 3.1 Full test suite
+
+```sh
+make check
+```
+
+Automake test rules discover built `test_programs` and execute each through the `RUN_TEST` wrapper.
+
+### 3.2 Per‑component tests
+
+```sh
+make -C src/lib check
+make -C src/lib-mail check
+make -C src/lib-regex check
+make -C src/lib-storage check
+```
+
+### 3.3 Valgrind / RUN_TEST wrapper behavior
+
+- If `valgrind` is installed, tests run under valgrind using a generated `run-test.sh` helper script.
+- If `valgrind` is **not** installed, or `NOVALGRIND=1` environment variable is set, tests run normally.
+- Because tests use `$(LIBTOOL) execute`, agents must install `libtool`.
+
+To disable valgrind usage on agents, ensure `NOVALGRIND=1` environment variable
+is set when running make check.
+
+---
+
+## 4. Creating commits
+
+- **Keep commits atomic.** Each commit should contain only the changes directly related to what the message describes—no unrelated fixes or mixed refactoring.
+- **Use imperative form in commit titles.** Example: *“Add foo to bar”*, not *“Added”* or *“Adds”*.
+- **Prefix the commit title with the affected component.**
+  Examples:
+  - `configure: Add libpcre2 detection`
+  - `lib-test: Add new test helper`
+- **Provide meaningful commit messages.** The body should explain *what* changed and *why*. Include details about new options, behaviors, or implications.
+- **For bug fixes**, describe the problem, symptoms (including crashes or assertion failures if applicable), and, when known, reference the commit that introduced the issue.
+- **One purpose per commit.** Avoid mixing multiple unrelated changes into a single commit.
+
+## 5. Notes for maintainers
+
+- Keep this document up to date when adding dependencies or new test modules.
+- Examples are intentionally minimal; adjust package lists per your CI baseline.
+