Audit Command::new sites for missing stdin(Stdio::null())

[Antoinegtir]

Scope

Currently every spawned tokio task that shells out (xcrun, adb, flutter) calls .stdin(Stdio::null()) individually. A few new code paths are missing it (grep for Command::new without .stdin). When missing, key strokes can leak into the child stdin and corrupt the TTY state.

What to do

• grep -n "Command::new" crates/ and find call sites without .stdin(Stdio::null()).
• Add .stdin(Stdio::null()) where the child does not legitimately need stdin (i.e. everywhere except the flutter daemon spawn that uses stdin for hot reload).
• Add a clippy::missing_const_for_fn-style lint? Probably not — too noisy. Just be diligent.

Acceptance

• No Command::new in crates/fl-cli/src/multi.rs is missing a .stdin(...) call.
• PR description lists every site touched.