feat(cli): cph completions <shell> 子命令(clap_complete)

用 clap_complete 4 给 cph 生成 shell 补全脚本(bash/zsh/fish/powershell/elvish)。
clap ValueEnum 枚举 Shell;run_completions 从 Cli::command() 派生,自动跟随
子命令/flag 演进。用法:`cph completions zsh > ~/.zfunc/_cph`。

- 依赖经 `cargo add clap_complete@4 -p cph-cli` 添加(4.6.5,默认 features 无额外依赖)
- README 补补全安装一节

测试:全 workspace 49 passed。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -294,6 +294,15 @@ dependencies = [
  "strsim",
 ]
 
+[[package]]
+name = "clap_complete"
+version = "4.6.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e0a7a9bfdb35811f9e59832f0f05975114d2251b415fb534108e6f34060fd772"
+dependencies = [
+ "clap",
+]
+
 [[package]]
 name = "clap_derive"
 version = "4.6.1"
@@ -399,6 +408,7 @@ name = "cph-cli"
 version = "0.0.1"
 dependencies = [
  "clap",
+ "clap_complete",
  "cph-check",
  "cph-diag",
  "cph-typst",