feat(i18n): add multilanguage support (en/zh-CN/ru)

Implement first-version multilanguage support for gdb-cli:

- Add i18n module with locale detection and translation function
- Support three languages: English, Simplified Chinese, Russian
- Locale precedence: GDB_CLI_LANG > system locale > default (en)
- Normalize locale aliases (en_US→en, zh_CN→zh-CN, ru_RU→ru)
- Fallback to English for unsupported locales/missing keys

New files:
- src/gdb_cli/i18n.py - Core i18n module
- src/gdb_cli/locales/ - Translation catalogs (en.py, zh_cn.py, ru.py)
- README.ru.md - Russian README
- tests/test_i18n.py - 22 i18n unit tests

Modified:
- cli.py - Integrate i18n for all command help texts
- README.md, README.zh-CN.md - Add Russian to language switcher

Tests: 245 passed (22 new i18n tests)

Constraint: Click evaluates help text at import time
Confidence: high
Scope-risk: narrow

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

Author: Cerdore

README.md

@@ -5,7 +5,7 @@
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](LICENSE)
 [![CI](https://github.com/Cerdore/gdb-cli/actions/workflows/ci.yml/badge.svg)](https://github.com/Cerdore/gdb-cli/actions/workflows/ci.yml)
 
-[English](README.md) | [中文](README.zh-CN.md)
+[English](README.md) | [中文](README.zh-CN.md) | [Русский](README.ru.md)
 
 A GDB debugging tool designed for AI Agents (Claude Code, etc.). Uses a "thin client CLI + GDB built-in Python RPC Server" architecture, enabling stateful GDB debugging through Bash.