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/bash/git-common.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# Git-specific common functions for the git extension.
+# Extracted from scripts/bash/common.sh — contains only git-specific
+# branch validation and detection logic.
+
+# Check if we have git available
+has_git() {
+    git rev-parse --show-toplevel >/dev/null 2>&1
+}
+
+# Validate that a branch name matches the expected feature branch pattern.
+# Accepts sequential (###-*) or timestamp (YYYYMMDD-HHMMSS-*) formats.
+check_feature_branch() {
+    local branch="$1"
+    local has_git_repo="$2"
+
+    # For non-git repos, we can't enforce branch naming but still provide output
+    if [[ "$has_git_repo" != "true" ]]; then
+        echo "[specify] Warning: Git repository not detected; skipped branch validation" >&2
+        return 0
+    fi
+
+    if [[ ! "$branch" =~ ^[0-9]{3}- ]] && [[ ! "$branch" =~ ^[0-9]{8}-[0-9]{6}- ]]; then
+        echo "ERROR: Not on a feature branch. Current branch: $branch" >&2
+        echo "Feature branches should be named like: 001-feature-name or 20260319-143022-feature-name" >&2
+        return 1
+    fi
+
+    return 0
+}