portable-pty and assert_cmd: drop or build the pty harness

[blooop]

Question

portable-pty = "0.9" sits in the workspace manifest used by no crate (absent from Cargo.lock); assert_cmd is a live dl dev-dependency with zero uses. The Rust pty harness they imply was never built, so Runner::session's real-terminal behavior and the skim picker's has-a-terminal branch are exercised only via the Python e2e suite (Docker required). Decide: drop the deps, or build the harness. Graduate the resulting build onto the map.

[blooop]

human-decided: Drop assert_cmd only, as asked and as answered.

The ticket's larger half was overtaken on main: since the review at d890ad1, rust/aid/tests/interactive.rs is a real portable-pty harness (landed with 8cd53ac "aid asks for the prompt while the workspace boots") and portable-pty is a live, used workspace dependency — dropping it would delete a working test suite. assert_cmd remains a dl dev-dependency with zero uses (rust/dl/Cargo.toml:58, no .rs reference anywhere) and goes.

A dl pty harness for Runner::session / the skim picker's tty branch is not built now: the aid harness is a ready template for the day a dl tty defect warrants one, and the Python e2e suite covers those branches today. Graduates one small build: remove the unused dev-dependency (the proof is the lockfile and a clean cargo test --workspace, not a behavior test — there is no behavior).