feat(website): Add last updated dates for agents, prompts, instructions, and skills

- Add git-dates.mjs utility to extract file modification dates from git history
- Include lastUpdated field in JSON data for all resource types
- Display relative time (e.g., '3 days ago') with full date on hover
- Add 'Recently Updated' sort option to agents, prompts, instructions, and skills pages
- Update deploy-website.yml to use fetch-depth: 0 for full git history

CI overhead: ~20-30s additional for full git checkout

eng/utils/git-dates.mjs

@@ -0,0 +1,103 @@
+#!/usr/bin/env node
+
+/**
+ * Utility to extract last modification dates from git history.
+ * Uses a single git log command for efficiency.
+ */
+
+import { execSync } from "child_process";
+import path from "path";
+
+/**
+ * Get the last modification date for all tracked files in specified directories.
+ * Returns a Map of file path -> ISO date string.
+ *
+ * @param {string[]} directories - Array of directory paths to scan
+ * @param {string} rootDir - Root directory for relative paths
+ * @returns {Map<string, string>} Map of relative file path to ISO date string
+ */
+export function getGitFileDates(directories, rootDir) {
+  const fileDates = new Map();
+
+  try {
+    // Get git log with file names for all specified directories
+    // Format: ISO date, then file names that were modified in that commit
+    const gitArgs = [
+      "--no-pager",
+      "log",
+      "--format=%aI", // Author date in ISO 8601 format
+      "--name-only",
+      "--diff-filter=ACMR", // Added, Copied, Modified, Renamed
+      "--",
+      ...directories,
+    ];
+
+    const output = execSync(`git ${gitArgs.join(" ")}`, {
+      encoding: "utf8",
+      cwd: rootDir,
+      stdio: ["pipe", "pipe", "pipe"],
+    });
+
+    // Parse the output: alternating date lines and file name lines
+    // Format is:
+    // 2026-01-15T10:30:00+00:00
+    //
+    // file1.md
+    // file2.md
+    //
+    // 2026-01-14T09:00:00+00:00
+    // ...
+
+    let currentDate = null;
+    const lines = output.split("\n");
+
+    for (const line of lines) {
+      const trimmed = line.trim();
+
+      if (!trimmed) {
+        continue;
+      }
+
+      // Check if this is a date line (ISO 8601 format)
+      if (/^\d{4}-\d{2}-\d{2}T/.test(trimmed)) {
+        currentDate = trimmed;
+      } else if (currentDate && trimmed) {
+        // This is a file path - only set if we haven't seen this file yet
+        // (first occurrence is the most recent modification)
+        if (!fileDates.has(trimmed)) {
+          fileDates.set(trimmed, currentDate);
+        }
+      }
+    }
+  } catch (error) {
+    // Git command failed - might not be a git repo or no history
+    console.warn("Warning: Could not get git dates:", error.message);
+  }
+
+  return fileDates;
+}
+
+/**
+ * Get the last modification date for a single file.
+ *
+ * @param {string} filePath - Path to the file (relative to git root)
+ * @param {string} rootDir - Root directory
+ * @returns {string|null} ISO date string or null if not found
+ */
+export function getGitFileDate(filePath, rootDir) {
+  try {
+    const output = execSync(
+      `git --no-pager log -1 --format="%aI" -- "${filePath}"`,
+      {
+        encoding: "utf8",
+        cwd: rootDir,
+        stdio: ["pipe", "pipe", "pipe"],
+      }
+    );
+
+    const date = output.trim();
+    return date || null;
+  } catch (error) {
+    return null;
+  }
+}