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/web/src/api/endpoints.ts

@@ -302,7 +302,7 @@ export const getUsage = (
     to?: string;
     groupBy: UsageGroupBy;
     agentId?: string;
-    /** Model filters are given as a pair (only takes effect when both provider and modelId are supplied). */
+    /** Model filter is always a whole pair — both fields or neither; a model is never referenced by id alone. */
     provider?: string;
     modelId?: string;
   },
@@ -330,6 +330,10 @@ export const listWorkspaceFiles = (sessionId: string, path: string) =>
 export const workspaceFileUrl = (sessionId: string, path: string, download = false): string =>
   `/api/sessions/${sessionId}/files/content?path=${encodeURIComponent(path)}${download ? "&download=1" : ""}`;
 
+/** Sandboxed top-level preview URL (open an html file in a new tab): real content type under a CSP sandbox, see the server route. */
+export const workspaceFilePreviewUrl = (sessionId: string, path: string): string =>
+  `/api/sessions/${sessionId}/files/content?path=${encodeURIComponent(path)}&preview=1`;
+
 export const uploadWorkspaceFile = (sessionId: string, path: string, dataBase64: string) =>
   apiFetch<void>(`/api/sessions/${sessionId}/files/content`, {
     method: "PUT",