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/core/src/environment/tools/run-subagent.ts

@@ -82,6 +82,15 @@ export function createSubagentTool(
       }
       const agentId = typeof args.agent_id === "string" ? args.agent_id : undefined;
       const modelId = typeof args.model_id === "string" ? args.model_id : undefined;
+      const provider = typeof args.provider === "string" ? args.provider : undefined;
+      // A model is referenced by the complete (provider, model_id) pair — never half of one.
+      // Caught here rather than in createSession so the model is told which half it left out.
+      if ((modelId === undefined) !== (provider === undefined)) {
+        yield* fail(
+          "[run_subagent error: `model_id` and `provider` must be given together (a model reference is the pair), or both omitted to use the Project default model]",
+        );
+        return { stopReason: "failed" };
+      }
       const yieldMs = clampYield(
         args.yield_time_ms,
         DEFAULT_SUBAGENT_YIELD_MS,
@@ -106,6 +115,7 @@ export function createSubagentTool(
         const handle = await runner.spawn({
           ...(agentId !== undefined ? { agentId } : {}),
           ...(modelId !== undefined ? { modelId } : {}),
+          ...(provider !== undefined ? { provider } : {}),
         });
         session = new ManagedSubagentSession(handle);
       } catch (err) {