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/README.md

@@ -37,7 +37,7 @@ pnpm --filter @prismshadow/penguin-server start   # node dist/index.js
 
 - **CSRF**: session cookie is `SameSite=Lax` and writes accept only `Content-Type: application/json`; no CSRF token yet.
 - **No login rate limiting**: add throttling at a reverse proxy for public deployments.
-- **Built-in admin starts as `admin` / `admin123`**: change it immediately (a banner keeps reminding until you do).
+- **Built-in admin starts as `admin` / `penguin-2026`**: change it immediately (a banner keeps reminding until you do).
 - Passwords use `node:crypto` scrypt (`scrypt$N$r$p$salt$hash`, timingSafeEqual); login sessions renew on a 7-day sliding window; the DB stores only the token's sha256.
 - Model credentials live in the Project's hidden 0600 config file; the API always masks them.
 - Behind a reverse proxy, disable response buffering for SSE paths (the server already sends `X-Accel-Buffering: no`) and forward `x-forwarded-proto` to enable Secure cookies.