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/docs/content/interfaces.en.md

@@ -89,7 +89,7 @@ interface GenerativeModelConfig {
 `GenerativeModel` (`packages/core/src/llm/generative-model.ts`) grounds the contract on the `AutoLLMClient` of the `@prismshadow/agenthub` model gateway:
 
 - the gateway maintains conversation history **statefully**, receiving only new messages each turn; resuming a Session replays committed history through a one-time `setHistory`;
-- an internal `EventTranslator` translates gateway stream events into `partial_*` fragments plus complete messages, preserving the `signature` / `phase` fidelity fields; complete messages settle in thinking → text → tool_call order;
+- an internal `EventTranslator` translates gateway stream events into `partial_*` fragments plus complete messages, preserving each item's opaque `fidelity` payload verbatim; segmentation mirrors the gateway's own aggregation — a thinking block is closed by its fidelity payload and a run of equal fidelity stays one block (OpenAI-compatible clients stamp every delta with the same `{ reasoning_field }`, which must not split blocks), while a text segment splits on a differing `fidelity.phase` and closes on a `fidelity.signature`, fidelity keys accumulating on merge; complete messages settle in thinking → text → tool_call order;
 - `ToolCallIdAllocator` disambiguates providers that use the function name as the call id (append `#n` inbound, strip outbound), scoped to the whole Session;
 - provider differences (tool-call formats, reasoning content, streaming events) are absorbed entirely inside the gateway — see [Models & Providers](/models).
 
@@ -169,7 +169,7 @@ A tool emits only content deltas; framing, timeouts, truncation, `stop_reason` p
 Human is deliberately not an interface class. The SDK caller *is* the Human:
 
 ```ts
-const session = await agent.createSession({ workspaceDir, modelId });
+const session = await agent.createSession({ workspaceDir, provider, modelId });
 
 session.run(
   newMessages: OmniMessage[],                    // input: the Prompt