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/src/auth/service.ts

@@ -3,7 +3,7 @@
  * login / logout / password change / session validation.
  *
  * - No open registration: on startup, if there are no users at all, the built-in
- *   admin `admin` is seeded (initial password admin123), and it adopts
+ *   admin `admin` is seeded (initial password penguin-2026), and it adopts
  *   `default_project`; all other users are created by an admin via the user
  *   backend (admin-service).
  * - An initial password (whether seeded or set by an admin) is flagged with
@@ -22,7 +22,7 @@ export const MIN_PASSWORD_LENGTH = 8;
 
 /** Built-in admin: user_id and initial password (matches the README and login-page hint). */
 export const ADMIN_USER_ID = "admin";
-export const ADMIN_INITIAL_PASSWORD = "admin123";
+export const ADMIN_INITIAL_PASSWORD = "penguin-2026";
 
 function sha256Hex(value: string): string {
   return createHash("sha256").update(value).digest("hex");