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/features/traces/trace-file-view.tsx

@@ -219,16 +219,18 @@ export function TraceFileView({
   }, [projectId, agentId, sessionId, index]);
 
   // Pricing for the session's Model (main session only; sub-session Tokens
-  // live in their own Trace and aren't part of this file):
-  // session_meta carries a paired reference (provider + model_id), matched against model config by that pair.
+  // live in their own Trace and aren't part of this file): session_meta carries a paired
+  // reference (provider + model_id), matched against model config by that whole pair.
+  // A Trace whose session_meta has no provider is legacy data (core refuses to resume it
+  // for the same reason) — the model_id alone identifies nothing, since the same id can
+  // exist under several providers at different prices, so no pricing is shown rather than
+  // an arbitrary first match.
   const pricing = useMemo(() => {
     const meta = events.find((m) => m.type === "session_meta");
     const ref = meta ? (meta.payload as { model_id?: string; provider?: string }) : undefined;
-    if (!ref?.model_id) return undefined;
-    return models?.models.find(
-      (m) =>
-        m.modelId === ref.model_id && (ref.provider === undefined || m.provider === ref.provider),
-    )?.pricing;
+    if (!ref?.model_id || !ref.provider) return undefined;
+    return models?.models.find((m) => m.modelId === ref.model_id && m.provider === ref.provider)
+      ?.pricing;
   }, [events, models]);
 
   const costOf = (b: Buckets): number | null => {