cli: fix zsh completion (#4692)

Before this change:
```
tamird@L03G26TD12 codex-rs % codex
zsh: do you wish to see all 3864 possibilities (1285 lines)?
```

After this change:
```
tamird@L03G26TD12 codex-rs % codex
app-server              -- [experimental] Run the app server
apply                a  -- Apply the latest diff produced by Codex agent as a `git apply` to your local working tree
cloud                   -- [EXPERIMENTAL] Browse tasks from Codex Cloud and apply changes locally
completion              -- Generate shell completion scripts
debug                   -- Internal debugging commands
exec                 e  -- Run Codex non-interactively
generate-ts             -- Internal: generate TypeScript protocol bindings
help                    -- Print this message or the help of the given subcommand(s)
login                   -- Manage login
logout                  -- Remove stored authentication credentials
mcp                     -- [experimental] Run Codex as an MCP server and manage MCP servers
mcp-server              -- [experimental] Run the Codex MCP server (stdio transport)
responses-api-proxy     -- Internal: run the responses API proxy
resume                  -- Resume a previous interactive session (picker by default; use --last to continue the most recent)
```

Author: tamird

codex-rs/exec/src/cli.rs

@@ -77,7 +77,7 @@ pub struct Cli {
 
     /// Initial instructions for the agent. If not provided as an argument (or
     /// if `-` is used), instructions are read from stdin.
-    #[arg(value_name = "PROMPT")]
+    #[arg(value_name = "PROMPT", value_hint = clap::ValueHint::Other)]
     pub prompt: Option<String>,
 }
 
@@ -99,7 +99,7 @@ pub struct ResumeArgs {
     pub last: bool,
 
     /// Prompt to send after resuming the session. If `-` is used, read from stdin.
-    #[arg(value_name = "PROMPT")]
+    #[arg(value_name = "PROMPT", value_hint = clap::ValueHint::Other)]
     pub prompt: Option<String>,
 }
 

codex-rs/tui/src/cli.rs

@@ -7,7 +7,7 @@ use std::path::PathBuf;
 #[command(version)]
 pub struct Cli {
     /// Optional user prompt to start the session.
-    #[arg(value_name = "PROMPT")]
+    #[arg(value_name = "PROMPT", value_hint = clap::ValueHint::Other)]
     pub prompt: Option<String>,
 
     /// Optional image(s) to attach to the initial prompt.