feat: introduce --compute-indices flag to codex-file-search (#1419)

This is a small quality-of-life feature, the addition of
`--compute-indices` to the CLI, which, if enabled, will compute and set
the `indices` field for each `FileMatch` returned by `run()`. Note we
only bother to compute `indices` once we have the top N results because
there could be a lot of intermediate "top N" results during the search
that are ultimately discarded.

When set, the indices are included in the JSON output when `--json` is
specified and the matching indices are displayed in bold when `--json`
is not specified.

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

@@ -20,6 +20,10 @@ pub struct Cli {
     #[clap(long, short = 'C')]
     pub cwd: Option<PathBuf>,
 
+    /// Include matching file indices in the output.
+    #[arg(long, default_value = "false")]
+    pub compute_indices: bool,
+
     // 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