#20745 base red: cmux-tui Windows build skips the unix-only other-app check (cx-zdbd0)

Fixes the feat base red in the cmux-tui Windows GNU build (bead cx-zdbd0). Other lanes: this is the cannot find app_identity in crate red on cmux-tui artifacts / build x86_64-pc-windows-gnu.

bc697beb0b57 made cli/wire/socket.rs call crate::app_identity on every platform (the other-app check, lines 79/85/91), but main.rs keeps mod app_identity behind #[cfg(unix)]. The module can't be un-gated cheaply because it reads the bundle's Info.plist through plist and cmux-agent-pane-policy, which are unix-only dependencies. A Windows cmux is never inside an app bundle, so in_other_apps_terminal could never be true there. This PR gates the check on unix and keeps other_app = false elsewhere. Nothing changes on unix.

Base pushes reuse the published tree and skip this build, so the red only shows on PRs that touch cmux-tui.

Red / green

cmux-tui artifacts / build x86_64-pc-windows-gnu

• base (on #20740, run 38639003378): error[E0433]: cannot find app_identity in crate at socket.rs:79, :85, :91
• this head: see the same job on this PR (it serves as the Windows cargo check)

Changelog

none

🤖 Generated with Claude Code

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Summary by cubic

Fixes the Windows GNU build of cmux-tui, which failed because cli/wire/socket.rs called crate::app_identity on every platform while main.rs only compiles that module on unix.

• Gates the other-app check behind #[cfg(unix)] and sets other_app = false on Windows, where a cmux is never inside an app bundle.
• Keeps the OTHER_APP_STATE_KEYS lookup unix-only so the non-unix path compiles.
• No behavior change on unix.