feat(ui): open Develop artifacts in the OS editor

Develop stays read-first: each artifact whose source is a repository
path gains an Open in editor action routed through a path-validated
main-process handler (shell.openPath), instead of an in-app write path.
Persona and prompt-assembly sources open the governing cordis.yml or
source file directly.

packages/ui/desktop/README.md

@@ -189,6 +189,6 @@ Start with a desktop package that defines shared UI contracts, then build the El
 
 - **Development build only** — this package ships a usable Electron/Vite app and a real ACP subprocess bridge, but it is not yet packaged as a signed distributable.
 - **ACP is the first runtime channel** — direct in-process embedding could make context queries and restarts richer, but would make isolation, teardown, and hot reload harder.
-- **Develop is read-first** — it exposes prompts, tools, plugins, config, runtime state, and the change loop as a source browser; direct graphical plugin/config editing is deferred.
+- **Develop is read-first** — it exposes prompts, tools, plugins, config, runtime state, and the change loop as a source browser; editing routes to the OS default editor through each artifact's `Open in editor` action (repository paths only), and in-app graphical editing stays deferred.
 - **Trace refresh is mixed live/persisted** — chat streams from ACP live updates (rendered incrementally, so composer input, fold state, and scroll survive streaming), while Trajectory and Waterfall read persisted JSONL after turns complete.
 - **Context and Compare surfaces are unimplemented** — the session view ships `Chat`, `Trajectory`, and `Waterfall`; the `Context`/`Compare` contracts above and replay remain documented product shape for later work.