refactor: extract shared read-file-to-string helper (63 occurrences)

[chaliy]

Severity: LOW | Type: Missing Utility | Effort: Small

Nearly every file-reading builtin repeats the same fs.read_file + String::from_utf8_lossy + error formatting pattern (63 occurrences).

Proposed fix: async fn read_text_file(fs, path, cmd_name) -> Result<String, ExecResult> shared helper.

Ref: specs/015-technical-debt.md #19

[chaliy]

Deferring: Replacing all 63 occurrences of the read_file + from_utf8_lossy pattern would be >500 lines of diff. The pattern also varies across builtins (some handle errors differently, some process the bytes before converting).

A helper can be added incrementally — each builtin PR can migrate its own call sites. The utility function could live in builtins/mod.rs as:

pub(crate) async fn read_text_file(fs: &Arc<dyn FileSystem>, path: &Path) -> Result<String> {
    let bytes = fs.read_file(path).await?;
    Ok(String::from_utf8_lossy(&bytes).into_owned())
}

But mass-replacing 63 sites in one PR is too large and error-prone.