[Minor] Add CLAUDE.md with development guidelines

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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+# Rspamd - Development Guide
+
+## Build & Test
+
+Use `/build-and-test` to build and run all unit tests. Manual steps:
+
+- Build directory: `~/rspamd.build` (out-of-source, never build in source tree)
+- Build: `cd ~/rspamd.build && ninja -j8 install`
+- C/C++ tests: `~/rspamd.build/test/rspamd-test-cxx`
+- Lua tests: `~/rspamd.build/test/rspamd-test -p /rspamd/lua`
+- Lua lint: `luacheck src/plugins/lua/ lualib/ rules/` (from project root)
+
+## Code Style
+
+### C/C++ (tabs, 4-wide)
+- Indentation: **tabs** (see `.clang-format`: `UseTab: ForContinuationAndIndentation`, `TabWidth: 4`)
+- Run `clang-format` using the project `.clang-format` before committing
+- Pointer alignment: right (`char *p`, not `char* p`)
+- Braces: opening brace on same line for control statements, new line after functions
+
+### Lua (2-space indent)
+- Indentation: **2 spaces**, no tabs
+- Run `luacheck` before committing any Lua changes
+
+## Language Standards
+
+### C++ (prefer for new code)
+- Standard: **C++20** (`CMAKE_CXX_STANDARD 20`)
+- Prefer C++ for all new code
+- Use `contrib/ankerl/unordered_dense.h` for hash maps/sets — do **not** use `std::unordered_map` or `std::unordered_set`
+
+### C (existing code)
+- Keep plain C style in existing C files
+- Use C++ only when writing new modules or substantially rewriting old ones
+
+### Lua (LuaJIT + modern Lua compatibility)
+- All Lua code must work with both **LuaJIT (5.1)** and **standard Lua 5.3, 5.4, and upcoming 5.5**
+- Avoid Lua 5.2+ only features (goto, bitwise operators syntax, integer division `//`) unless guarded
+- Use `bit` module or compat shims for bitwise operations
+- Do not rely on LuaJIT-only FFI in core code paths unless a pure-Lua fallback exists
+
+## Performance
+
+- Focus on performance; use efficient algorithms and data structures
+- Prefer `ankerl::unordered_dense` over std hash containers
+- Minimize allocations in hot paths
+- Profile before optimizing — measure, don't guess
+
+## Printf Differences
+
+Rspamd has its own printf implementation (`src/libutil/printf.h`) that differs from GNU printf. Key format specifiers:
+
+| Format | Type | Notes |
+|--------|------|-------|
+| `%s` | `const char *` | null-terminated string |
+| `%*s` | `int len, const char *` | length-prefixed string |
+| `%v` | `GString *` | GLib string |
+| `%V` | `rspamd_fstring_t *` | Rspamd fstring |
+| `%T` | `rspamd_ftok_t` | Rspamd token |
+| `%e` | `GError *` | GLib error |
+| `%xs` | string | hex-encoded output |
+| `%bs` | string | base32-encoded output |
+| `%Bs` | string | base64-encoded output |
+| `%z` | `ssize_t/size_t` | with optional `u`, `x`, `X`, `h`, `H` modifiers |
+| `%d` | `int` | with optional `u`, `x`, `X`, `h`, `H`, `b`, `B` modifiers |
+| `%l` | `long` | with optional modifiers |
+| `%D` | `int32_t/uint32_t` | with optional modifiers |
+| `%L` | `int64_t/uint64_t` | with optional modifiers |
+
+**In Lua**: `rspamd_logger` uses `%s` as the universal placeholder for all argument types (not `%d`, `%f`, etc.).
+
+Read `src/libutil/printf.h` before writing any C/C++ logging or formatting code.
+
+## Commit Messages
+
+Format: `[Tag] Description`
+
+Tags: `[Feature]`, `[Fix]`, `[CritFix]`, `[Minor]`, `[Project]`, `[Rework]`, `[Conf]`, `[Test]`, `[Rules]`
+
+All commits must be GPG-signed (`git commit -S`).