feat(work): add ISO timestamps + auto-bump on staged work-doc changes

- New scripts/work/bump-updated-timestamps.mjs stamps the `updated:`
  frontmatter field to the current ISO 8601 UTC timestamp on every
  staged docs/work/**/*.md file. Idempotent; adds the field after
  `created:` if missing.
- .husky/pre-commit invokes the bump script as step 2 (before
  rebuild-state) so _state.json sees the fresh timestamp.
- Backfill all existing work docs (4 PRDs + 3 epics + 21 stories):
    * created: promoted from \`YYYY-MM-DD\` -> ISO timestamp using
      git log --diff-filter=A on each file (first-commit date for
      stories that had no \`created:\` line, midnight UTC for PRDs
      and epics that had date-only created).
    * updated: added from \`git log -1 --format=%aI\` on each file
      (last-commit timestamp); will be re-stamped to "now" by the
      pre-commit hook on this commit.

Stories that had no \`created:\` line now get one.

scripts/work/bump-updated-timestamps.mjs

@@ -0,0 +1,78 @@
+#!/usr/bin/env node
+/**
+ * Stamp every staged `docs/work/**\/*.md` file's frontmatter `updated:` field
+ * to the current ISO 8601 timestamp. Runs from `.husky/pre-commit` before
+ * `pnpm work rebuild-state`, so `_state.json` sees the fresh value.
+ *
+ * Idempotent: re-running produces the same result. The script silently no-ops
+ * on files without frontmatter or without an `updated:` line to replace —
+ * adds the field after `created:` if missing.
+ *
+ * Only stamps files explicitly listed in the staged diff; never walks the
+ * tree. That way the timestamp tracks "the last commit that actually
+ * modified the file," not "the last commit period."
+ */
+import { execSync } from "node:child_process";
+import fs from "node:fs";
+import path from "node:path";
+
+const REPO_ROOT = execSync("git rev-parse --show-toplevel", {
+  encoding: "utf8",
+}).trim();
+
+function stagedWorkDocs() {
+  const out = execSync("git diff --cached --name-only --diff-filter=ACMR", {
+    cwd: REPO_ROOT,
+    encoding: "utf8",
+  });
+  return out
+    .split("\n")
+    .map((p) => p.trim())
+    .filter(
+      (p) =>
+        p.startsWith("docs/work/") &&
+        p.endsWith(".md") &&
+        !p.endsWith("README.md"),
+    );
+}
+
+function stampUpdated(content, isoNow) {
+  const fmMatch = content.match(/^(---\n)([\s\S]+?)(\n---)/);
+  if (!fmMatch) return content;
+  const [full, openDelim, body, closeDelim] = fmMatch;
+
+  let newBody;
+  if (/^updated:\s*/m.test(body)) {
+    newBody = body.replace(/^updated:\s*.*$/m, `updated: ${isoNow}`);
+  } else if (/^created:\s*/m.test(body)) {
+    // Insert `updated:` immediately after the `created:` line.
+    newBody = body.replace(/^(created:\s*.*)$/m, `$1\nupdated: ${isoNow}`);
+  } else {
+    return content;
+  }
+  return content.replace(full, `${openDelim}${newBody}${closeDelim}`);
+}
+
+function main() {
+  const files = stagedWorkDocs();
+  if (files.length === 0) return;
+  const isoNow = new Date().toISOString();
+  let touched = 0;
+  for (const rel of files) {
+    const abs = path.join(REPO_ROOT, rel);
+    if (!fs.existsSync(abs)) continue;
+    const original = fs.readFileSync(abs, "utf8");
+    const next = stampUpdated(original, isoNow);
+    if (next === original) continue;
+    fs.writeFileSync(abs, next);
+    execSync(`git add ${JSON.stringify(rel)}`, { cwd: REPO_ROOT });
+    touched++;
+  }
+  if (touched > 0) {
+    console.log(
+      `bump-updated-timestamps: stamped ${touched} file(s) at ${isoNow}`,
+    );
+  }
+}
+
+main();