feat: standalone file search CLI (#1386)

Standalone fuzzy filename search library that should be helpful in
addressing https://github.com/openai/codex/issues/1261.

codex-rs/file-search/src/cli.rs

@@ -0,0 +1,38 @@
+use std::num::NonZero;
+use std::path::PathBuf;
+
+use clap::ArgAction;
+use clap::Parser;
+
+/// Fuzzy matches filenames under a directory.
+#[derive(Parser)]
+#[command(version)]
+pub struct Cli {
+    /// Whether to output results in JSON format.
+    #[clap(long, default_value = "false")]
+    pub json: bool,
+
+    /// Maximum number of results to return.
+    #[clap(long, short = 'l', default_value = "64")]
+    pub limit: NonZero<usize>,
+
+    /// Directory to search.
+    #[clap(long, short = 'C')]
+    pub cwd: Option<PathBuf>,
+
+    // While it is common to default to the number of logical CPUs when creating
+    // a thread pool, empirically, the I/O of the filetree traversal offers
+    // limited parallelism and is the bottleneck, so using a smaller number of
+    // threads is more efficient. (Empirically, using more than 2 threads doesn't seem to provide much benefit.)
+    //
+    /// Number of worker threads to use.
+    #[clap(long, default_value = "2")]
+    pub threads: NonZero<usize>,
+
+    /// Exclude patterns
+    #[arg(short, long, action = ArgAction::Append)]
+    pub exclude: Vec<String>,
+
+    /// Search pattern.
+    pub pattern: Option<String>,
+}