Changelog, dev startup, README, AgentHub 0.4.0, model catalog, and landing site (#7)

Branch-length batch covering tooling, the model layer, the Web App and the public
surfaces. Highlights:

- Changelog: a per-release `changelog/<version>/` tree, grouped by the surface each
  change touches, with a root CHANGELOG.md holding one line per release.
- Dev startup: `scripts/dev-prebuild.mjs` serializes the skills+core prebuild behind a
  lock and keeps `pnpm install` current; `pnpm dev` runs server+web together.
- AgentHub 0.3.3 -> 0.4.0: OmniMessage complete payloads carry one opaque `fidelity`
  object in place of item-level `signature`/`phase`, threaded verbatim through Trace,
  replay and resume; malformed classification adapted to the new error types.
- Model layer: a model is always referenced by an explicit `(provider, model_id)` pair.
  The provider is never inferred, guessed or defaulted -- both the catalog inference and
  the unique-match config resolution are gone, and CLI, SDK, server routes and
  run_subagent all require the complete pair. Catalog gains the Qwen Token Plan, Qwen
  Pay-As-You-Go and Fireworks AI gateways, plus an expanded OpenRouter group.
- Web App: catalog preset sync and per-group speed test on the Models page, positional
  slash commands, a markdown renderer, skill-library update reminders, and a vertically
  centred draft page whose upward menus size themselves to the room available.
- Public surfaces: restructured READMEs, the penguin.ooo landing site and blog, refreshed
  benchmark results for both suites, and the demo videos playing on the landing page.

Includes the fixes from a full review of the branch: 23 confirmed findings, among them a
provider-inference bug that could send one vendor's API key to another vendor's endpoint,
and an Escape handler that destroyed the composer's contents unrecoverably.

Verified on the branch head: pnpm test (1127 passing, 7 packages), pnpm typecheck and
pnpm format:check clean, Playwright e2e 14/14.

packages/server/test/title-generator.test.ts

@@ -164,6 +164,17 @@ describe("title-generator", () => {
     await waitFor(() => recorded.length > 0);
   });
 
+  it("fallback strips a leading <use_skills> block so the marker never becomes the title", async () => {
+    const calls = { count: 0, args: [] as unknown[] };
+    const gen = makeGenerator();
+    gen.maybeGenerate(CTX, fakeSession({ title: null, usage: null }, calls), {
+      fallbackText: "<use_skills>\nskills: web-design\n</use_skills>\n做一个落地页",
+    });
+    await waitFor(() => sessions.findById(ROW.sessionId)?.title !== null);
+    // Not "<use_skills>" — the marker block is removed before the first line is taken.
+    expect(sessions.findById(ROW.sessionId)?.title).toBe("做一个落地页");
+  });
+
   it("LLM returns null and the fallback material is blank → the title stays NULL (retryable next time)", async () => {
     const calls = { count: 0, args: [] as unknown[] };
     const gen = makeGenerator();