Windows support for @claude (binary discovery, sessions, spawning)

[sam-powers]

Releases are macOS-only today (README rationale): the editor itself builds and runs on Windows, but the @claude integration doesn't, so we don't ship installers that can't deliver the full experience. This issue tracks what it would take to lift that.

What's Unix-specific

All in src-tauri/src/lib.rs:

1. Binary discovery — resolve_claude_binary()

   • Step 1 shells out to which claude; Windows needs where.exe (or Command::new("claude.cmd") probing).
   • Step 2's candidate list is built from $HOME (%USERPROFILE% on Windows) with Unix paths (~/.claude/local/claude, ~/.local/bin/claude, /opt/homebrew/bin, ~/.nvm/versions/node/*/bin). Windows equivalents: %USERPROFILE%\.local\bin\claude.exe, npm global prefix (%APPDATA%\npm\claude.cmd), nvm-windows dirs.
   • Step 3 falls back to $SHELL -lic 'command -v claude' — no Windows analog; would be skipped or replaced with a where.exe pass.
   • On Windows the npm-installed CLI is a claude.cmd shim, which Command::new won't execute directly — it needs cmd /C or resolution to the underlying node script.

2. Session discovery — claude_projects_dir() and the list_claude_sessions / find_session_for_markdown / check_session_compacted family — these use dirs::home_dir().join(".claude/projects"), which should resolve on Windows, but the session-path encoding (project dirs derived from absolute paths) needs verification against how Claude Code encodes C:\... paths on Windows.

3. Process spawning — spawn_claude_resume — spawned children should set CREATE_NO_WINDOW (creation_flags(0x08000000)) so a console window doesn't flash; cancellation in ChildRegistry uses kill(), which is fine cross-platform via std.

4. The Claude Code plugin (plugin/quill-integration) uses open "quill://…" — Windows would use start "" "quill://…" (and Quill's deep-link registration via tauri-plugin-deep-link should already handle the registry side).

Acceptance

• @claude replies and AI tracked-changes work on a Windows machine with Claude Code installed via the native installer and via npm.
• Release workflow re-adds the windows-latest target.
• README/USER_GUIDE install matrix updated.