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/session-index.test.ts

@@ -103,6 +103,21 @@ describe("session-index", () => {
     expect(list.sessions.map((s) => s.sessionId)).toContain(session.sessionId);
   });
 
+  it("half a model reference is 400: the missing half is never inferred", async () => {
+    await configureModels();
+    // Only modelId: even though it names the one configured model, the provider is never
+    // filled in for the caller — a reference is submitted as a pair or not at all.
+    const onlyModel = await api.post(base(), { modelId: "claude-sonnet-4-6" });
+    expect(onlyModel.status).toBe(400);
+    const onlyProvider = await api.post(base(), { provider: "anthropic" });
+    expect(onlyProvider.status).toBe(400);
+    // The complete pair works, and so does omitting both (Project default).
+    expect(
+      (await api.post(base(), { provider: "anthropic", modelId: "claude-sonnet-4-6" })).status,
+    ).toBe(201);
+    expect((await api.post(base(), {})).status).toBe(201);
+  });
+
   it("an explicit Workspace only needs to exist; it may live outside the Project directory", async () => {
     await configureModels();
     const inside = path.join(t.root, projectId, "my-workdir");