Add Windows support (full parity) — discussion + tracking

[nachiket0310]

Summary

flow is macOS-only today. This issue proposes — and tracks — a
full-parity Windows port, opened per CONTRIBUTING's "discuss before
sending code" rule for out-of-scope platforms.

Current state

There are no build tags and no runtime.GOOS branching anywhere in
the tree; the session-spawn layer is entirely AppleScript-based.

Won't compile for GOOS=windows:

• internal/app/auto.go — syscall.SysProcAttr{Setsid: true},
  proc.Signal(syscall.Signal(0)), syscall.EPERM
• internal/app/owner_tick.go — syscall.SysProcAttr{Setsid: true}

Compiles but fails at runtime:

• Process-table scans via ps -axo (iterm.go, terminal.go,
  harness/claude/claude.go) — powers the live-session guard + focus
• Terminal backends are all osascript (iterm/terminal/warp/ghostty);
  warp also uses macOS open
• spawner.Detect() falls through to iTerm for unknown env
• ShellQuote is POSIX single-quote; EncodeCwd assumes /-paths
• Owners scheduler doc covers launchd/systemd/cron only

In our favor: pure-Go SQLite (no CGO → trivial cross-compile),
filepath.Join + os.UserHomeDir() throughout, clean Harness
interface, and FLOW_TERM=bg background mode already bypasses
terminals entirely.

Proposed plan

• Phase 0 — compile on Windows: extract the two Setsid/liveness
  sites behind a setDetached/processAlive seam (proc_unix.go /
  proc_windows.go). Add a GOOS=windows cross-compile CI gate.
• Phase 1 — background mode runs on Windows: OS-aware
  spawner.Detect(), Windows-correct EncodeCwd, notepad editor
  default.
• Phase 2 — interactive backend: internal/winterm driving
  Windows Terminal (wt.exe); Windows-aware ShellQuote.
• Phase 3 — process-scan parity (replace ps -axo).
• Phase 4 — distribution (flow-windows-{amd64,arm64}.exe),
  Task Scheduler for owners, windows-latest in CI, scope-statement
  updates (README / CONTRIBUTING / CLAUDE.md).

Open questions

• How does Claude Code encode a Windows cwd into
  ~/.claude/projects/<dir>? EncodeCwd must match exactly — needs
  empirical verification on Windows.
• Does the SessionStart hookCommand fire correctly on Windows?
  (Must not rename it — orphans existing installs.)
• Live-session guard: full cmdline scan vs. graceful degrade for MVP?