feat: create extensions/git with manifest, commands, scripts, and auto-install in init

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>
Agent-Logs-Url: https://github.com/github/spec-kit/sessions/809a1dbf-1301-4312-b4d2-e18f9b3e8b2f

extensions/git/scripts/powershell/git-common.ps1

@@ -0,0 +1,33 @@
+#!/usr/bin/env pwsh
+# Git-specific common functions for the git extension.
+# Extracted from scripts/powershell/common.ps1 — contains only git-specific
+# branch validation and detection logic.
+
+function Test-HasGit {
+    try {
+        git rev-parse --show-toplevel 2>$null | Out-Null
+        return ($LASTEXITCODE -eq 0)
+    } catch {
+        return $false
+    }
+}
+
+function Test-FeatureBranch {
+    param(
+        [string]$Branch,
+        [bool]$HasGit = $true
+    )
+    
+    # For non-git repos, we can't enforce branch naming but still provide output
+    if (-not $HasGit) {
+        Write-Warning "[specify] Warning: Git repository not detected; skipped branch validation"
+        return $true
+    }
+    
+    if ($Branch -notmatch '^[0-9]{3}-' -and $Branch -notmatch '^\d{8}-\d{6}-') {
+        Write-Output "ERROR: Not on a feature branch. Current branch: $Branch"
+        Write-Output "Feature branches should be named like: 001-feature-name or 20260319-143022-feature-name"
+        return $false
+    }
+    return $true
+}