fix: correct variable scope in PowerShell deduplication and update docs

- Fix Update-IfNew in PowerShell update-agent-context.ps1
  - Changed from $script: scope to Set-Variable -Scope 1
  - Properly mutates parent function's local variables
  - Fixes deduplication tracking for shared AGENTS.md file
  - Prevents incorrect default Claude file creation

- Update create-release-packages.sh documentation
  - Add missing 'forge' to AGENTS list in header comment
  - Documentation now matches actual ALL_AGENTS array

Without this fix, AGENTS.md would be updated multiple times (once
for each agent sharing it: opencode, codex, amp, kiro, bob, pi, forge)
and the script would always create a default Claude file even when
agent files exist.

scripts/powershell/update-agent-context.ps1

@@ -447,8 +447,9 @@ function Update-AllExistingAgents {
         }
         
         # Record the file as seen before attempting the update
-        $script:updatedPaths += $realPath
-        $script:found = $true
+        # Use parent scope (1) to modify Update-AllExistingAgents' local variables
+        Set-Variable -Name updatedPaths -Value ($updatedPaths + $realPath) -Scope 1
+        Set-Variable -Name found -Value $true -Scope 1
         
         # Perform the update
         return (Update-AgentFile -TargetFile $FilePath -AgentName $AgentName)