feat: OpenDesign (nexu-io/open-design) compatibility — MCP today, native runtime adapter upstream

OpenDesign (github.com/nexu-io/open-design, Apache-2.0, 93k★) is an open-source Claude-Design-alternative studio that turns coding-agent CLIs into design engines: prototypes, decks, design systems (DESIGN.md), plugins, HTML/PDF/PPTX/MP4 export. It integrates agents two ways (their `docs/agent-adapters.md`):

1. **MCP path (cheap, mostly docs):** `od mcp install ` writes the agent's MCP config for OD's stdio server (project/design-system/template listing, live file reads). Codewhale's MCP client + self-serve MCP auth (#5747) should already talk to it. Work: verify our side end-to-end, add a docs page (`docs/mcp.md` section or `docs/design.md`) with the manual config while `od mcp install codewhale` doesn't exist, and open a small upstream PR adding Codewhale to their install list.

2. **Native runtime path (upstream PR):** OpenDesign can *launch and stream* supported agents (`apps/daemon/src/runtimes/defs/`): structured streaming, model discovery, cancellation, session resume. A Codewhale runtime definition there makes Codewhale a first-class design engine in their Studio. Requires on our side: a headless/print invocation mode with machine-readable events if `codewhale exec` doesn't already cover it — audit before the PR.

Why bother: it gives Codewhale users a design-studio surface we don't have to build (design systems, deck/prototype templates, artifact preview/export), and it's a distribution channel — their compatibility table lists 26 CLIs and Codewhale isn't on it.

Future design-tooling angle (founder): their DESIGN.md design-system format + design-templates + MCP (agents read live design files) could feed our Tideline/web design work — e.g. the shell canvas round-trip. Any adoption must respect per-directory licenses (their bundled skills carry their own; provenance lives in their `docs/references.md`).

Sequencing: not 0.9.12 scope. After release: (a) verify MCP interop + docs, (b) upstream runtime-def PR, (c) evaluate DESIGN.md for our design pipeline.

Start by verifying the MCP path and reviewing docs/mcp.md or docs/design.md for the compatibility documentation. Audit whether `codewhale exec` already provides the required headless, machine-readable events, then inspect OpenDesign’s `apps/daemon/src/runtimes/defs/` before considering the upstream runtime PR. Done means MCP interoperability is verified and the documentation, upstream install entry, and runtime work are clearly sequenced.

Written by the indexing model from the issue text.