fix(deps): drop unmaintained atty crate

[amitksingh1490]

Why

Resolves Dependabot alert #8 (GHSA-g98v-hv3f-hcfr).

The atty crate is unmaintained (last release ~3 years ago) and carries a low-severity advisory for a potential unaligned read on Windows. Upstream has not published a patched version and is unlikely to, so the advisory explicitly recommends replacing it.

Why this approach

std::io::IsTerminal has been stable since Rust 1.70 and provides exactly the TTY-detection capability we were using atty for. Leaning on the standard library is preferable to pulling in another third-party crate (e.g. is-terminal) because it:

• Removes a security-flagged dependency without introducing a new one.
• Reduces the dependency surface and build time.
• Is maintained by the Rust project itself, so future soundness concerns are covered upstream.

Scope

atty had exactly one caller in the workspace (stdin pipe detection in forge_main). Behavior is preserved: piped stdin still populates the piped-input path, and interactive TTY sessions still bypass the stdin read.

Verification

• cargo tree -i atty --workspace confirms the crate is no longer in the graph.
• cargo check passes.
• Behavior was validated end-to-end via tmux (TTY path) and multiple pipe variants (non-TTY path): empty pipe, single-line, multi-line, and /dev/null redirect. Interactive TUI launches and exits cleanly; piped commands do not hang.

Co-Authored-By: ForgeCode noreply@forgecode.dev