fix(#345): restore prom-client, harden normalizer against ReDoS, strip frontmatter (review round 1)

Addresses the round-1 review of #369:

F1 [CRITICAL] Restore prom-client. The prior commit removed it as a 'stray dep',
but metrics.registry.ts imports it unconditionally at startup (main.ts boot), so
a clean frozen install had no prom-client -> server tsc TS2307 + boot crash. It
was surviving only via hoisting from a warm store. Restored to apps/server
dependencies + regenerated the lock (prom-client/tdigest/bintrees return),
keeping the @docmost/prosemirror-markdown dep. Verified: clean frozen install ->
require.resolve('prom-client') ok, server tsc EXIT 0.

F2 [HIGH] Two quadratic ReDoS vectors in foreign-markdown.ts on untrusted import
(runs synchronously on the request thread, 30MB cap):
  (a) pass-2 was O(lines x defs) — a per-def RegExp rebuilt and run over every
      line. Replaced with ONE precompiled alternation regex over all def ids,
      built once per document, with an id->body lookup in the replacer: O(text).
  (b) the inline-code split alternation backtracks quadratically on a long
      UNCLOSED backtick run. Lines over 8KB now skip the split (left untouched) —
      a real footnote line is never that long.

F3 [WARNING] Restore the leading YAML front-matter strip that the retired
markdownToHtml layer did. Without it, Obsidian/Hugo/Jekyll/git-sync files leak
their front-matter into the body (and 'title:' renders as a setext heading that
title extraction can hijack).

F4 [WARNING] Extend the zip-import spec with an image (width+align) + callout
fidelity assertion through the PM->HTML->PM hop (the one hop the package suite
does not cover).

F5/F6 Update AGENTS.md (apps/server is now a prosemirror-markdown consumer) and
make the server pretest build prosemirror-markdown too.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/integrations/import/utils/foreign-markdown.spec.ts

@@ -83,6 +83,49 @@ describe('normalizeForeignMarkdown — GFM reference footnotes', () => {
     const callouts = ':::info\nHi\n:::\n\n> [!warning]\n> Careful';
     expect(normalizeForeignMarkdown(callouts)).toBe(callouts);
   });
+
+  it('strips a leading YAML front-matter block (Obsidian/Hugo/git-sync files)', () => {
+    const out = normalizeForeignMarkdown(
+      '---\ntitle: My Page\ntags: [a, b]\n---\n\n# Heading\n\nBody.',
+    );
+    expect(out).toBe('# Heading\n\nBody.');
+    // The front-matter must not leak into the body as a setext heading.
+    expect(out).not.toContain('title: My Page');
+    expect(out).not.toContain('---');
+  });
+
+  it('does not strip a horizontal rule that is not leading front-matter', () => {
+    const md = 'Intro paragraph.\n\n---\n\nAfter the rule.';
+    expect(normalizeForeignMarkdown(md)).toBe(md);
+  });
+
+  it('is linear on a document with thousands of definitions (no quadratic blowup)', () => {
+    // F2(a): the pass-2 rewrite must be O(text), not O(text × defs). Build a
+    // pathological doc (many defs + many plain text lines) and assert it
+    // completes well under a second — a quadratic implementation took ~14s.
+    const N = 4000;
+    const refs = Array.from({ length: N }, (_, i) => `line ${i} plain text`).join('\n');
+    const defs = Array.from({ length: N }, (_, i) => `[^n${i}]: def ${i}`).join('\n');
+    const doc = `start[^n0] and[^n${N - 1}] end\n\n${refs}\n\n${defs}`;
+    const t0 = Date.now();
+    const out = normalizeForeignMarkdown(doc);
+    const elapsed = Date.now() - t0;
+    expect(elapsed).toBeLessThan(2000);
+    // Sanity: the two real references were still inlined.
+    expect(out).toContain('^[def 0]');
+    expect(out).toContain(`^[def ${N - 1}]`);
+  });
+
+  it('is bounded on a long unclosed backtick run (no inline-split ReDoS)', () => {
+    // F2(b): a huge unterminated backtick run must not cause quadratic
+    // backtracking in the inline-code split. Oversized lines skip the split
+    // entirely (left untouched), so this returns promptly.
+    const line = 'x' + '`'.repeat(200000);
+    const doc = `${line}\n\n[^1]: def`;
+    const t0 = Date.now();
+    normalizeForeignMarkdown(doc);
+    expect(Date.now() - t0).toBeLessThan(2000);
+  });
 });
 
 describe('foreign markdown import acceptance (normalizer + canonical parser)', () => {