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>

crates/cph-cli/Cargo.toml

@@ -13,3 +13,4 @@ cph-check = { path = "../cph-check" }
 cph-diag = { workspace = true }
 cph-typst = { path = "../cph-typst" }
 clap = { version = "4", features = ["derive"] }
+clap_complete = "4"

crates/cph-cli/src/main.rs

@@ -46,6 +46,22 @@ enum Command {
         #[arg(short = 'o', long, value_name = "OUT")]
         out: Option<PathBuf>,
     },
+    /// Print a shell-completion script to stdout (clap_complete; ADR-0013 opt-in
+    /// sibling: a local convenience, no lesson involved). Pipe to your shell's
+    /// completion file, e.g. `cph completions zsh > ~/.zfunc/_cph`.
+    Completions {
+        /// Which shell to generate completions for.
+        shell: Shell,
+    },
+}
+
+#[derive(Debug, Clone, Copy, clap::ValueEnum)]
+enum Shell {
+    Bash,
+    Zsh,
+    Fish,
+    PowerShell,
+    Elvish,
 }
 
 fn main() -> ExitCode {
@@ -59,9 +75,29 @@ fn main() -> ExitCode {
     match cli.command {
         Command::Check { path } => run_check(&path, &engine),
         Command::Build { path, target, out } => run_build(&path, &engine, &target, out),
+        Command::Completions { shell } => run_completions(shell),
     }
 }
 
+/// Emit a shell-completion script for `shell` to stdout. The script is built
+/// from the same `Cli` clap definition above, so it tracks subcommands/flags as
+/// they evolve.
+fn run_completions(shell: Shell) -> ExitCode {
+    use clap::CommandFactory;
+    use clap_complete::Shell as CompShell;
+
+    let sh = match shell {
+        Shell::Bash => CompShell::Bash,
+        Shell::Zsh => CompShell::Zsh,
+        Shell::Fish => CompShell::Fish,
+        Shell::PowerShell => CompShell::PowerShell,
+        Shell::Elvish => CompShell::Elvish,
+    };
+    let mut cmd = Cli::command();
+    clap_complete::generate(sh, &mut cmd, "cph", &mut std::io::stdout());
+    ExitCode::SUCCESS
+}
+
 /// Print every diagnostic in `report` to stderr, followed by a summary line.
 fn print_diagnostics(report: &CheckReport) {
     for d in &report.diagnostics {