Fix/windows terminal shell

[xiaomo-xty]

What & why

Kyde currently reaches several Unix-only assumptions when built and used on Windows:

• The embedded terminal reads $SHELL and falls back to /bin/zsh, which cannot spawn on Windows.
• The app-level ConfirmKey binding consumes Enter before the focused terminal can submit a command.
• The single-instance implementation uses Unix-domain sockets.
• Update verification and tests assume shasum, Unix executable bits, LF line endings, or / path separators.

This PR makes those paths portable:

• Use %COMSPEC% for Windows terminal sessions, with cmd.exe as a fallback.
• Add a terminal-scoped Enter action that writes \r to the PTY, plus a GPUI dispatch regression test.
• Preserve executable modes from Git's index on Windows and make line-ending/path assertions platform-independent.
• Compute update SHA-256 checksums in-process. sha2 is already in Kyde's normal dependency graph.
• Compile the Unix-socket single-instance guard only on Unix; Windows keeps the previous multi-instance behavior until it has native IPC.

How to test

On Windows:

1. Put the Windows SDK directory containing fxc.exe on PATH.
2. Run cargo build.
3. Run cargo test --all.
4. Open a repository, toggle the terminal, enter echo hello, and press Enter.
5. Partially stage an executable file and confirm its index mode is preserved.

Checks completed on Windows:

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test --all
• cargo build

Speed & footprint

• No new work added to a hot path (per-keystroke highlight, per-frame render, per-file-select diff) - or if there is, it's justified and has a perf_* guard test.
• No new always-on dependency that bloats the binary / resident RAM. sha2 was already present in Kyde's normal dependency graph.
• Startup still feels instant (the app should open before you finish letting go of the mouse).
• N/A - docs / chore / pure refactor with no runtime impact.

Checklist

• cargo build is clean (no new warnings).
• cargo test is green (cargo test perf if you touched a hot path).
• No vendor code or assets copied from Zed or any GPL source (patterns only - see README -> Reference material).
• CHANGELOG.md is updated; README.md / CLAUDE.md do not need changes for this behavior.

[themis-gatacre-games]

AI Review: Ship it

A solid portability PR that makes Kyde's terminal, single-instance guard, update checksum, and tests work on Windows without regressing Unix. The changes are well-scoped: each platform fork is cleanly gated with #[cfg], the in-process SHA-256 computation is correct (verified the hardcoded test digest), and the new TerminalEnter action + gpui dispatch test is a good catch for a real key-swallowing bug.

Correctness	5/5
Test coverage	4/5
Code quality	5/5
Product impact	4/5

Walkthrough

• Terminal shell selection - default_shell() now reads %COMSPEC% on Windows (falling back to cmd.exe) instead of the Unix-only $SHELL / /bin/zsh.
• Enter key relay - New TerminalEnter action bound in the Terminal context shadows the app-root ConfirmKey, preventing Enter from being swallowed; includes a gpui dispatch regression test.
• Single-instance guard - instance module and all its call sites are #[cfg(unix)]-gated; Windows keeps multi-instance behavior until native IPC is added.
• In-process SHA-256 - verify_checksum now uses the sha2 crate (already in the dep graph) instead of shelling out to shasum, making updates work on Windows.
• Git mode preservation - stage_content_mode on non-Unix reads the mode from git ls-files --stage instead of always returning 100644, preserving executable bits through partial staging.
• Test portability - core.autocrlf=false in git test fixtures, PathBuf-based assertions in kyde-tree, #[cfg(unix)] on symlink-dependent shellcmd tests.

How to verify

1. On Windows: open a repo, toggle the terminal (Ctrl+``), type echo hello`, press Enter - the command should execute (not be swallowed).
2. On macOS/Linux: same test - confirm no regression in terminal Enter behavior.
3. On Windows: partially stage an executable file (git update-index --chmod=+x first), confirm the staged entry keeps 100755 mode.
4. Trigger a self-update on Windows and confirm the SHA-256 verification passes (no shasum dependency).
5. On macOS/Linux: confirm the single-instance guard still works (kyde . in a second terminal should activate the existing window).

Automate: cargo test --all on both Windows and Linux/macOS CI runners.

Product take: This unblocks Windows as a real target. The Enter-key fix is the highest-value change - without it the embedded terminal is unusable. The rest is necessary plumbing to get cargo build and cargo test green on Windows. Solid infrastructure work.

Assumptions & unverified claims

• The sha2 crate is stated to already be in Kyde's dependency graph (via a transitive dep); I verified it appears in Cargo.lock but did not confirm it adds zero binary size overhead.
• The PR claims all four checks (fmt, clippy, test, build) pass on Windows; I reviewed the code but could not run a Windows build to confirm.
• alacritty_terminal's Shell::new with cmd.exe on Windows is assumed to work correctly (the PTY options include a Windows-only escape_args: true field, suggesting alacritty has Windows support).

A PR that teaches an app to speak both /bin/zsh and cmd.exe - truly bilingual. reviewed at 7807cc2