Fix Windows support: platform-abstract process lifecycle, binary update, and install script

[danbao]

Problem

WeClaw builds for Windows (windows/amd64, windows/arm64) in CI, but several core functions fail at runtime or compile-time on Windows:

Issue	Location	Impact
syscall.SIGINT, syscall.SIGTERM in signal.NotifyContext	start.go	SIGTERM does not exist on Windows — compilation fails or behavior is undefined
Signal(0) for process detection	start.go processExists()	On Windows Signal(0) always returns nil — process detection never works
syscall.SIGTERM for process termination	start.go stopAllWeclaw(), restart.go	Windows has no SIGTERM — process termination silently fails
pkill -f for cleanup	start.go stopAllWeclaw()	Windows has no pkill — orphan cleanup never works
replaceBinary() rename	update.go	Running .exe is file-locked on Windows — weclaw update always fails
getLatestVersion() uses GitHub API	update.go	Anonymous rate limit (60/hr) — fails in CI or with frequent checks
No Windows install script	—	Users must manually download and configure PATH

Solution

1. Platform abstraction via build tags (proc_unix.go / proc_windows.go)

Move all platform-specific logic into build-tagged files. The existing files only had setSysProcAttr — now they also provide:

Function	Unix	Windows
notifyContext()	SIGINT + SIGTERM	os.Interrupt
processExists()	Signal(0)	tasklist /FI "PID eq X"
signalTerminate()	SIGTERM	Kill() (hard kill)
killByName()	pkill -f	taskkill /F /IM

2. Remove syscall from shared code

start.go and restart.go no longer import syscall. All platform calls go through the abstracted functions above.

3. replaceBinary() — Windows file-lock handling

On Windows, a running .exe cannot be overwritten. The new logic:

1. Rename current binary to .old
2. Place new binary at original path
3. If step 2 fails, restore .old back

4. getLatestVersion() — avoid GitHub API rate limits

Replace api.github.com/repos/.../releases/latest (JSON, 60 req/hr anonymous limit) with HTTP redirect from github.com/.../releases/latest (no rate limit, no JSON parsing).

5. install.ps1 — Windows installer

New PowerShell install script that:

• Detects architecture (amd64/arm64)
• Downloads latest release via redirect
• Installs to $env:LOCALAPPDATA\weclaw
• Adds to user PATH if not present

Files Changed

• cmd/proc_unix.go — Added notifyContext, processExists, signalTerminate, killByName
• cmd/proc_windows.go — Added Windows-specific implementations for all 5 platform functions
• cmd/start.go — Removed syscall import; use notifyContext, signalTerminate, killByName
• cmd/restart.go — Removed syscall import; use signalTerminate
• cmd/update.go — getLatestVersion via redirect; replaceBinary with Windows file-lock handling
• install.ps1 — New Windows installer script

Testing

• go build ./... passes (darwin)
• GOOS=windows GOARCH=amd64 go build ./... passes (cross-compile)
• go test -race ./... all pass