Add sync agent instructions script

sync-agent-instructions.ps1

@@ -0,0 +1,79 @@
+param(
+    [string]$Remote = "gitea",
+    [string]$MainBranch = "main"
+)
+
+$ErrorActionPreference = "Stop"
+
+# Verify remote exists.
+$remotes = git remote
+if ($remotes -notcontains $Remote) {
+    throw "Git remote '$Remote' does not exist. Available remotes: $($remotes -join ', ')"
+}
+
+Write-Host "Using remote: $Remote"
+Write-Host "Canonical branch: $MainBranch"
+
+# Update remote references.
+git fetch $Remote --prune
+
+# Update local main from the selected remote.
+git switch $MainBranch
+git pull --ff-only $Remote $MainBranch
+
+# Get all branches on this specific remote.
+$remotePrefix = "$Remote/"
+
+$branches = git for-each-ref `
+    --format="%(refname:short)" `
+    "refs/remotes/$Remote" |
+    Where-Object {
+        $_ -ne "$Remote/$MainBranch" -and
+        $_ -ne "$Remote/HEAD"
+    }
+
+foreach ($remoteBranch in $branches) {
+    # Remove "<remote>/" prefix while preserving branch names such as
+    # agent/foo/bar.
+    $branch = $remoteBranch.Substring($remotePrefix.Length)
+
+    Write-Host ""
+    Write-Host "Updating $branch..."
+
+    # Check whether the local branch already exists.
+    git show-ref --verify --quiet "refs/heads/$branch"
+
+    if ($LASTEXITCODE -eq 0) {
+        git switch $branch
+
+        # Make sure the local branch reflects the latest remote branch
+        # before modifying the instruction files.
+        git pull --ff-only $Remote $branch
+    }
+    else {
+        git switch --track -c $branch "$Remote/$branch"
+    }
+
+    # Force these files to exactly match canonical main.
+    git restore `
+        --source "$MainBranch" `
+        -- AGENTS.md PROJECT_GOAL.md
+
+    git add AGENTS.md PROJECT_GOAL.md
+
+    # Commit only when one of the files actually changed.
+    git diff --cached --quiet
+
+    if ($LASTEXITCODE -ne 0) {
+        git commit -m "Sync agent instructions from $MainBranch"
+        git push $Remote $branch
+    }
+    else {
+        Write-Host "Already up to date."
+    }
+}
+
+git switch $MainBranch
+
+Write-Host ""
+Write-Host "Finished synchronizing AGENTS.md and PROJECT_GOAL.md."