fix(process): 支持外部恢复命令提示

[alupao]

Summary / 概要

Allows an owning wrapper to provide one validated resume command. When CLAUDEX_RESUME_HINT is present, Claudex keeps its PTY proxy active, suppresses Claude Code’s raw footer, and prints only the wrapper-owned hint. Standalone Claudex behavior is unchanged.

Related Issues / 关联 Issue

Closes #14.

Type of Change / 变更类型

• Bug fix / 缺陷修复
• New feature / 新功能
• New provider support / 新增提供商
• Breaking change / 破坏性变更
• Documentation / 文档
• Refactor / 重构

Test Plan / 测试计划

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test — 344 unit tests and 1 integration test
• cargo check
• Live wrapper validation in normal and TERM=dumb terminal modes

Checklist / 检查清单

• cargo fmt --check passes
• cargo clippy — zero warnings
• cargo test — all tests pass
• Documentation updated (not applicable: opt-in wrapper integration, no standalone UX change)
• No sensitive data (API keys, tokens) committed