fix: don't write launcher PID into agent.pid on Windows

start-local-agent.ps1 was writing the wrapper PowerShell's PID into
agent.pid before node even started. The agent's ensureSingleInstance()
in server.ts then read that PID, saw the wrapper was still alive
(naturally — it's node's parent), and aborted with "Another Screen
Pilot agent is already running". server.ts writes its own real PID
after binding the port, so the ps1 has no business pre-writing it.

Also fix log encoding: replace '*>> \$LogFile' with
'2>&1 | Out-File -Encoding utf8 -Append' so agent.log is clean UTF-8
instead of UTF-16 LE.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ouyyyyang

scripts/dev/start-local-agent.ps1

@@ -201,7 +201,7 @@ $serverCommand = @(
     "`$env:SCREEN_PILOT_PID_FILE='$PidFile'"
     "`$env:SCREEN_PILOT_PORT_FILE='$PortFile'"
     "Set-Location '$RootDir'"
-    "& '$nodeAbsolute' build/node/core/agent/src/server.js *>> '$LogFile'"
+    "& '$nodeAbsolute' build/node/core/agent/src/server.js 2>&1 | Out-File -FilePath '$LogFile' -Append -Encoding utf8"
 ) -join "; "
 
 $process = Start-Process `
@@ -211,7 +211,10 @@ $process = Start-Process `
     -WindowStyle Hidden `
     -PassThru
 
-Set-Content -Path $PidFile -Value $process.Id
+# Do NOT write $process.Id to $PidFile here — $process is the launcher
+# PowerShell wrapper, not the node agent. server.ts writes its own real
+# PID once it binds the port. Writing the wrapper PID makes the agent's
+# ensureSingleInstance() see a "live" PID and exit immediately.
 
 Write-Host "Starting agent..."
 for ($attempt = 0; $attempt -lt 30; $attempt += 1) {