Add Blaxel sandbox blueprint and ecosystem entry

[astrobot-houston]

Originally proposed by @mstolarzblaxelai in #287. Their description is reproduced verbatim below.

All community-submitted pull requests are automatically converted to issues (bugs) & discussions (feature requests, enhancements) where they can be triaged and prioritized. Once prioritized, a PR implementation is created automatically.

Summary:

• Add a Blaxel sandbox blueprint that adapts an application-owned @blaxel/core SandboxInstance into Flue's SandboxApi.
• Add Blaxel ecosystem docs/catalog/sidebar entries.
• Cover Blaxel add/update behavior in the CLI local-registry tests.

Blocked:

• Local full CLI package test has an unrelated flue dev watcher failure: EMFILE: too many open files, watch; add/update, blueprint parser, typecheck, docs build, and www build are green.

---

Original implementation from #287 by @mstolarzblaxelai

[mstolarzblaxelai]

Thanks, happy to continue here. The implementation in #287 adds only a blueprint/docs/catalog entry for an application-owned Blaxel SandboxInstance; happy to adjust scope or provide more adapter rationale if useful.